United States Medical Licensing Examination (USMLE) Step 1 — Questions and Answers

Question 1: A patient with COPD has pCO2 of 55 mmHg and bicarbonate of 32 mEq/L. What is the acid-base disturbance?

Acute respiratory acidosis
Metabolic alkalosis with respiratory compensation
Chronic respiratory acidosis with metabolic compensation (Correct answer)
Mixed metabolic and respiratory acidosis

Correct answer: Chronic respiratory acidosis with metabolic compensation

Chronic respiratory acidosis shows elevated pCO2 (hypoventilation) with appropriately elevated bicarbonate (renal compensation of 3.5 mEq/L per 10 mmHg rise in pCO2), distinguishing chronic from acute forms.

Question 2: Choose the correct answer

Von Gierke disease (Correct answer)
Niemann-Pick disease
McArdle disease
Pompe disease
Gaucher disease

Correct answer: Von Gierke disease

Von Gierke disease (Type I glycogen storage disease) is caused by a deficiency in glucose-6-phosphatase, an enzyme crucial for gluconeogenesis and glycogenolysis. This leads to severe fasting hypoglycemia, lactic acidosis, hyperlipidemia, and hepatomegaly due to glycogen accumulation. Patients often present with a "doll-like" face and growth retardation.

Question 3: Which finding on renal biopsy is pathognomonic for Goodpasture syndrome?

Linear IgG deposits along the glomerular basement membrane (Correct answer)
Subepithelial humps
Electron-dense deposits in the subendothelial space
Granular IgG deposits in the mesangium

Correct answer: Linear IgG deposits along the glomerular basement membrane

Goodpasture syndrome shows linear IgG deposits along the GBM on immunofluorescence due to anti-GBM antibodies targeting type IV collagen (alpha-3 chain).

Question 4: Which enzyme deficiency causes cystinuria by impairing renal tubular reabsorption of dibasic amino acids?

Gamma-cystathionase
Cystine reductase
Defective cystine transporter (rBAT) (Correct answer)
Cystathionine beta-synthase

Correct answer: Defective cystine transporter (rBAT)

Cystinuria is caused by a defective cystine/dibasic amino acid transporter (rBAT) in renal tubules and intestine, leading to cystine stones.

Question 5: A young woman presents with a butterfly rash, arthritis, and proteinuria. ANA is positive. Which antibody is most specific for SLE?

Anti-histone
Anti-Smith (Correct answer)
Anti-dsDNA
Anti-SSA (Ro)

Correct answer: Anti-Smith

Anti-Smith (anti-Sm) antibody is the most specific antibody for SLE, while anti-dsDNA is highly specific but also fluctuates with disease activity.

Question 6: A patient on warfarin develops supratherapeutic INR after starting a new antibiotic. Which mechanism best explains this interaction?

Inhibition of CYP2C9 by the antibiotic (Correct answer)
Induction of CYP3A4
Displacement from plasma proteins
Direct inhibition of clotting factors

Correct answer: Inhibition of CYP2C9 by the antibiotic

Many antibiotics inhibit CYP2C9, which metabolizes warfarin, leading to decreased warfarin clearance and elevated INR.

Question 7: Choose the correct answer

Ventral horn
Dorsal horn
Dorsal column (Correct answer)

Correct answer: Dorsal column

The dorsal columns (fasciculus gracilis and fasciculus cuneatus) are ascending sensory tracts in the spinal cord. They are responsible for transmitting fine touch, vibration, and proprioception from the ipsilateral side of the body. Damage to the dorsal columns, as seen in conditions like tabes dorsalis or vitamin B12 deficiency, leads to impaired sensation in these modalities.

Question 8: Choose the correct answer

isolated gonadotropin deficiency
azoospermia
varicocele
germinal cell aplasia
immotile sperm (Correct answer)

Correct answer: immotile sperm

Immotile sperm, or asthenozoospermia, is a cause of male infertility where sperm are unable to move effectively, hindering their ability to reach and fertilize an egg. This can be due to structural defects in the sperm flagella (e.g., primary ciliary dyskinesia), metabolic deficiencies, or other genetic factors affecting sperm motility. Diagnosis involves semen analysis.

Question 9: Choose the correct answer

expansion of a trinucleotide repeat
random inactivation of the X chromosome
uniparental disomy (Correct answer)
a point mutation in an autosome
confined placental mosaicism

Correct answer: uniparental disomy

Uniparental disomy (UPD) occurs when an individual receives both copies of a chromosome, or part of a chromosome, from a single parent. If the chromosome involved carries imprinted genes, UPD can lead to genetic disorders like Prader-Willi or Angelman syndrome, even without a mutation, due to incorrect dosage of imprinted genes.

Question 10: Which diuretic acts on the thick ascending limb of the loop of Henle and inhibits the Na-K-2Cl cotransporter?

Acetazolamide
Hydrochlorothiazide
Spironolactone
Furosemide (Correct answer)

Correct answer: Furosemide

Furosemide inhibits the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle, producing powerful natriuresis and diuresis.

Question 11: A 45-year-old woman with rheumatoid arthritis develops dry eyes, dry mouth, and parotid enlargement. Antibodies to SSA (Ro) and SSB (La) are positive. What is the diagnosis?

Amyloidosis
Sarcoidosis
Sjögren syndrome (Correct answer)
SLE

Correct answer: Sjögren syndrome

Sjögren syndrome is an autoimmune exocrinopathy characterized by lymphocytic infiltration of lacrimal and salivary glands, causing sicca symptoms (keratoconjunctivitis sicca and xerostomia) with anti-SSA/SSB antibodies.

Question 12: A patient presents with inability to initiate arm abduction—unable to abduct beyond 15 degrees—after an anterior shoulder dislocation. Which muscle was most likely injured?

Teres minor
Infraspinatus
Subscapularis
Supraspinatus (Correct answer)

Correct answer: Supraspinatus

The supraspinatus initiates the first 15 degrees of arm abduction by stabilizing the glenohumeral joint; injury prevents initiation of abduction even though the deltoid remains intact.

Question 13: A patient with cirrhosis develops jaundice, ascites, and confusion. Asterixis is noted on exam. Serum ammonia is elevated. What is this complication called?

Spontaneous bacterial peritonitis
Wernicke encephalopathy
Hepatic encephalopathy (Correct answer)
Hepatorenal syndrome

Correct answer: Hepatic encephalopathy

Hepatic encephalopathy results from liver failure allowing ammonia and other toxins to reach the brain, causing asterixis (flapping tremor), confusion, and altered consciousness.

Question 14: A patient with HIV on antiretroviral therapy develops central adiposity, peripheral fat wasting, and hyperlipidemia. Which drug class causes this metabolic syndrome?

Protease inhibitors (Correct answer)
Integrase inhibitors
NNRTIs
NRTIs

Correct answer: Protease inhibitors

Protease inhibitors cause lipodystrophy syndrome characterized by central fat accumulation, peripheral lipoatrophy, and dyslipidemia.

Question 15: Which tumor marker is elevated in hepatocellular carcinoma?

CEA
PSA
Alpha-fetoprotein (AFP) (Correct answer)
CA 19-9

Correct answer: Alpha-fetoprotein (AFP)

AFP is the primary tumor marker for hepatocellular carcinoma (HCC) and is also elevated in yolk sac tumors and in normal fetal liver.

Question 16: A patient with suspected meningitis undergoes lumbar puncture. CSF shows low glucose, high protein, and predominantly lymphocytes. What is the most likely etiology?

Bacterial meningitis
Tuberculous meningitis (Correct answer)
Viral (aseptic) meningitis
Fungal meningitis

Correct answer: Tuberculous meningitis

Tuberculous meningitis typically shows low glucose, high protein, and lymphocytic pleocytosis in CSF, along with a subacute course, compared to the acute neutrophilic presentation of bacterial meningitis.

Question 17: Choose the correct answer

Hypertensive nephropathy
Diabetic nephropathy
Minimal change disease
Anti-glomerular basement membrane disease (Correct answer)
Lupus nephritis

Correct answer: Anti-glomerular basement membrane disease

Anti-glomerular basement membrane (GBM) disease, also known as Goodpasture syndrome, is an autoimmune disorder characterized by antibodies targeting type IV collagen in the glomerular and often alveolar basement membranes. This leads to rapidly progressive glomerulonephritis and pulmonary hemorrhage, presenting as hemoptysis and acute kidney injury.

Question 18: Which antifungal drug inhibits 14-alpha demethylase (CYP51) to block ergosterol synthesis?

Caspofungin
Fluconazole (Correct answer)
Flucytosine
Amphotericin B

Correct answer: Fluconazole

Fluconazole and other azole antifungals inhibit 14-alpha demethylase (CYP51), blocking the conversion of lanosterol to ergosterol in the fungal cell membrane.

Question 19: Which form of cell death is characterized by cell shrinkage, nuclear condensation (pyknosis), membrane blebbing, and formation of apoptotic bodies without inflammation?

Autophagy
Necrosis
Apoptosis (Correct answer)
Necroptosis

Correct answer: Apoptosis

Apoptosis is programmed cell death characterized by cell shrinkage, chromatin condensation, membrane blebbing, and apoptotic body formation without triggering inflammation.

Question 20: Orotic aciduria with megaloblastic anemia but no hyperammonemia suggests deficiency of which enzyme?

Ornithine transcarbamylase
Dihydroorotase
Carbamoyl phosphate synthetase II
UMP synthase (UMPS) (Correct answer)

Correct answer: UMP synthase (UMPS)

UMP synthase deficiency impairs pyrimidine synthesis, causing orotic acid accumulation and megaloblastic anemia without urea cycle disruption or hyperammonemia.

Question 21: Choose the correct answer

Esophageal body distal to the diaphragm
Esophageal body proximal to the diaphragm
Pharynx
Lower esophageal sphincter (Correct answer)
Upper esophageal sphincter

Correct answer: Lower esophageal sphincter

The lower esophageal sphincter (LES) is a physiological sphincter, not an anatomical one, formed by the muscularis propria of the distal esophagus and the surrounding diaphragm. Its primary function is to prevent reflux of gastric contents into the esophagus. Dysfunction of the LES is a common cause of gastroesophageal reflux disease (GERD).

Question 22: Choose the correct answer

Heroin overdose
Diabetic hyperosmolar coma
Diabetic ketoacidosis (Correct answer)

Correct answer: Diabetic ketoacidosis

Diabetic ketoacidosis (DKA) is a severe complication of diabetes characterized by hyperglycemia, metabolic acidosis, and ketonemia. It occurs due to profound insulin deficiency, leading to increased lipolysis and production of ketone bodies. These ketones accumulate, causing a drop in blood pH and resulting in symptoms like Kussmaul respirations, fruity breath, and altered mental status.

Question 23: Which drug is a selective serotonin reuptake inhibitor (SSRI) with the longest half-life and is used to minimize withdrawal syndrome?

Escitalopram
Paroxetine
Sertraline
Fluoxetine (Correct answer)

Correct answer: Fluoxetine

Fluoxetine has the longest half-life (~1-4 days) among SSRIs due to its active metabolite norfluoxetine, making discontinuation syndrome rare.

Question 24: A patient presents with ascending weakness that started in the feet after a recent GI illness. CSF shows elevated protein with normal cell count (albuminocytologic dissociation). What is the diagnosis?

Multiple sclerosis
Myasthenia gravis
Amyotrophic lateral sclerosis
Guillain-Barré syndrome (GBS) (Correct answer)

Correct answer: Guillain-Barré syndrome (GBS)

GBS is an acute inflammatory demyelinating polyneuropathy triggered by molecular mimicry after infection (often Campylobacter jejuni), showing characteristic albuminocytologic dissociation in CSF.

Question 25: A patient develops a 'winged scapula' following a radical mastectomy with axillary dissection. Which nerve was most likely damaged during the procedure?

Thoracodorsal nerve
Accessory nerve (CN XI)
Long thoracic nerve (Correct answer)
Medial pectoral nerve

Correct answer: Long thoracic nerve

The long thoracic nerve innervates the serratus anterior; its injury during mastectomy or axillary dissection causes winging of the scapula as the medial border protrudes from the thoracic wall.

Question 26: Choose the correct answer

Right ventricle (Correct answer)
Right atrium
Upper lobe of the left lung

Correct answer: Right ventricle

This question likely refers to the chamber of the heart that pumps deoxygenated blood into the pulmonary circulation. The right ventricle receives deoxygenated blood from the right atrium and then pumps it through the pulmonary artery to the lungs for oxygenation. Conditions like pulmonary hypertension or tricuspid regurgitation can lead to right ventricular hypertrophy or failure.

Question 27: Choose the correct answer

Paraseptal
Interstitial
Panacinar (Correct answer)

Correct answer: Panacinar

Panacinar emphysema, also known as panlobular emphysema, is a type of chronic obstructive pulmonary disease (COPD) characterized by the destruction of the entire acinus, from the respiratory bronchioles to the alveoli. This form of emphysema is strongly associated with alpha-1 antitrypsin deficiency. The widespread destruction of alveolar walls leads to enlarged airspaces and reduced gas exchange capacity throughout the lung.

Question 28: A patient has a positive D-dimer test and imaging confirms pulmonary embolism. Which ventilation-perfusion change occurs in the affected lung segment?

Decreased V/Q ratio (shunt physiology)
Decreased ventilation with normal perfusion
Increased V/Q ratio (dead space physiology) (Correct answer)
Normal V/Q ratio

Correct answer: Increased V/Q ratio (dead space physiology)

PE blocks perfusion to a ventilated lung segment, increasing V/Q ratio toward infinity (dead space effect), wasting ventilation without gas exchange.

Question 29: A 60-year-old man presents with lower back pain, hypercalcemia, anemia, and lytic bone lesions. Serum protein electrophoresis shows an M-spike. What is the diagnosis?

Paget disease of bone
Metastatic prostate cancer
Multiple myeloma (Correct answer)
Primary hyperparathyroidism

Correct answer: Multiple myeloma

Multiple myeloma presents with CRAB criteria: hyperCalcemia, Renal failure, Anemia, and Bone lytic lesions, with an M-protein spike on SPEP from clonal plasma cell proliferation.

Question 30: Choose the correct answer

Endometrial stromal sarcoma
Malignant mixed mullerian tumor (Correct answer)
Endolymphatic stromal myosis
Leiomyosarcoma
Endometrial carcinoma

Correct answer: Malignant mixed mullerian tumor

Malignant mixed mullerian tumor (MMMT), also known as carcinosarcoma, is a highly aggressive uterine malignancy containing both malignant epithelial (carcinomatous) and malignant mesenchymal (sarcomatous) components. It typically presents in postmenopausal women with abnormal uterine bleeding and has a poor prognosis due to its aggressive nature and high recurrence rate.

Question 31: A 65-year-old man has a prostate biopsy showing Gleason grade 4+5=9. What does this score indicate?

Benign prostatic hyperplasia
Intermediate-grade tumor
Well-differentiated tumor with low metastatic potential
Poorly differentiated tumor with high metastatic potential (Correct answer)

Correct answer: Poorly differentiated tumor with high metastatic potential

A Gleason score of 9 (grade group 5) represents the most poorly differentiated prostate cancer with the highest metastatic potential and worst prognosis.

Question 32: A sputum culture grows gram-positive cocci in clusters that are catalase-positive and coagulase-positive. What organism is this?

Streptococcus pyogenes
Staphylococcus aureus (Correct answer)
Enterococcus faecalis
Staphylococcus epidermidis

Correct answer: Staphylococcus aureus

Coagulase-positive Staphylococcus aureus is distinguished from other staphylococci by its production of coagulase, which converts fibrinogen to fibrin.

Question 33: A patient presents with weakness of hip flexion and knee extension along with loss of sensation over the anterior thigh and medial leg. Which nerve is most likely damaged?

Sciatic nerve
Obturator nerve
Lateral femoral cutaneous nerve
Femoral nerve (Correct answer)

Correct answer: Femoral nerve

The femoral nerve innervates the iliopsoas (hip flexion) and quadriceps (knee extension), and its saphenous branch provides sensation to the anterior thigh and medial leg.

Question 34: Choose the correct answer

Congenital heart disease
Rheumatic fever
Rheumatoid arthritis
Illicit drug use (Correct answer)
Systemic lupus erythematosus

Correct answer: Illicit drug use

Illicit intravenous drug use is a significant risk factor for infective endocarditis, particularly affecting the tricuspid valve. Contaminated needles introduce bacteria, commonly Staphylococcus aureus, into the bloodstream, which then colonize the heart valves. This can lead to vegetation formation, valve damage, and septic emboli.

Question 35: In DNA replication, which enzyme removes the RNA primer and replaces it with DNA?

DNA polymerase I (Correct answer)
Primase
DNA ligase
DNA polymerase III

Correct answer: DNA polymerase I

DNA polymerase I has 5'→3' exonuclease activity to remove RNA primers and 5'→3' polymerase activity to fill the gaps with DNA.

Question 36: A 5-year-old child has recurrent bacterial infections since 6 months of age, absent B cells, and very low immunoglobulin levels. The cellular immune system is intact. What is the diagnosis?

Severe combined immunodeficiency (SCID)
CVID
X-linked agammaglobulinemia (Bruton's) (Correct answer)
DiGeorge syndrome

Correct answer: X-linked agammaglobulinemia (Bruton's)

Bruton's agammaglobulinemia is an X-linked disorder caused by BTK mutation, preventing B cell maturation; patients have absent B cells, low IgG/IgA/IgM, and intact T cell immunity.

Question 37: A patient with atrial fibrillation is started on warfarin. Target INR range for non-valvular AF is:

2.0-3.0 (Correct answer)
3.0-4.0
1.5-2.0
2.5-3.5

Correct answer: 2.0-3.0

For non-valvular atrial fibrillation, the target INR is 2.0-3.0; this range balances stroke prevention with bleeding risk; mechanical heart valves require a higher INR of 2.5-3.5.

Question 38: Virchow's triad describes risk factors for thrombosis. Which factor represents endothelial injury?

Damaged vessel wall (Correct answer)
Turbulent blood flow
Hypercoagulability
Stasis of blood flow

Correct answer: Damaged vessel wall

Virchow's triad consists of endothelial injury (damaged vessel wall), abnormal blood flow (stasis/turbulence), and hypercoagulability; endothelial damage exposes subendothelial collagen.

Question 39: Choose the correct answer

bcl-2 activation (Correct answer)
bcr-c-abl
c-myc activation

Correct answer: bcl-2 activation

Bcl-2 is an anti-apoptotic protein whose activation or overexpression inhibits programmed cell death. This prolonged cell survival is a hallmark of many cancers, particularly follicular lymphoma, where a t(14;18) translocation leads to BCL2 overexpression. Therefore, bcl-2 activation contributes to oncogenesis by preventing the natural elimination of abnormal cells.

Question 40: Which of the following structures is located in the posterior mediastinum?

Thoracic duct (Correct answer)
Phrenic nerve
Superior vena cava
Thymus gland

Correct answer: Thoracic duct

The thoracic duct ascends through the posterior mediastinum alongside the descending thoracic aorta, esophagus, and azygos vein before draining into the left subclavian vein.

Question 41: Choose the correct answer

Propranolol (Correct answer)
Disopyramide
Amiodarone
Lidocaine
Verapamil

Correct answer: Propranolol

Propranolol is a non-selective beta-blocker (Class II antiarrhythmic) that reduces heart rate and contractility, thereby decreasing myocardial oxygen demand. It is used to treat various arrhythmias, including supraventricular tachycardias, and to prevent recurrent myocardial infarction. Its effects are mediated by blocking beta-adrenergic receptors in the heart.

Question 42: Which of the following structures does NOT pass through the carpal tunnel?

Flexor digitorum superficialis tendons
Flexor pollicis longus tendon
Median nerve
Ulnar nerve (Correct answer)

Correct answer: Ulnar nerve

The ulnar nerve passes through Guyon's canal (ulnar canal) at the wrist, not the carpal tunnel; the carpal tunnel contains the median nerve and 9 flexor tendons (4 FDS, 4 FDP, 1 FPL).

Question 43: The blood-testis barrier, which protects developing germ cells from immune surveillance, is formed by tight junctions between which cells?

Leydig cells
Myoid cells
Primary spermatocytes
Sertoli cells (Correct answer)

Correct answer: Sertoli cells

Sertoli cells form tight junctions with each other to create the blood-testis barrier, separating developing spermatocytes from the systemic immune system.

Question 44: Lesch-Nyhan syndrome is caused by deficiency of which enzyme?

Purine nucleoside phosphorylase
Xanthine oxidase
HGPRT (hypoxanthine-guanine phosphoribosyltransferase) (Correct answer)
Adenosine deaminase

Correct answer: HGPRT (hypoxanthine-guanine phosphoribosyltransferase)

Lesch-Nyhan syndrome results from HGPRT deficiency, impairing purine salvage and leading to gout, intellectual disability, and self-mutilation.

Question 45: Which HIV protein integrates viral cDNA into the host genome?

gp120
Integrase (IN) (Correct answer)
Protease
Reverse transcriptase

Correct answer: Integrase (IN)

HIV integrase catalyzes the integration of double-stranded viral cDNA into the host cell genome, creating the provirus; it is the target of drugs like raltegravir and dolutegravir.

Question 46: A patient presents with rice-water stools and rapid dehydration. The causative organism produces a toxin that permanently activates adenylate cyclase. What organism is responsible?

Salmonella typhi
Vibrio cholerae (Correct answer)
ETEC
Campylobacter jejuni

Correct answer: Vibrio cholerae

Vibrio cholerae produces cholera toxin (CT) that ADP-ribosylates Gs-alpha, permanently activating adenylate cyclase and causing massive chloride secretion and profuse watery diarrhea.

Question 47: Choose the correct answer

Epstein-Barr virus (EBV)
Hepatitis B virus (HBV) (Correct answer)
Human herpesvirus type 8 (HHV 8)

Correct answer: Hepatitis B virus (HBV)

Hepatitis B virus (HBV) is a well-established risk factor for hepatocellular carcinoma (HCC). Chronic HBV infection leads to persistent inflammation, hepatocyte damage, and regeneration, which increases the risk of genetic mutations and malignant transformation. The viral DNA can also integrate into the host genome, further contributing to oncogenesis.

Question 48: A patient develops 'red man syndrome' during IV vancomycin infusion. What is the underlying mechanism?

IgE-mediated type I hypersensitivity
Complement activation
Direct mast cell degranulation (non-IgE mediated) (Correct answer)
Immune complex deposition

Correct answer: Direct mast cell degranulation (non-IgE mediated)

Vancomycin causes non-immunologic direct mast cell degranulation, releasing histamine and causing flushing/erythema; slowing the infusion rate prevents this reaction.

Question 49: Which mechanism explains why aminoglycosides are bactericidal despite being protein synthesis inhibitors?

They cause misreading of mRNA, producing abnormal proteins that disrupt cell membranes (Correct answer)
They prevent binding of aminoacyl-tRNA
They bind the 50S subunit and block translocation
They inhibit peptidyl transferase activity

Correct answer: They cause misreading of mRNA, producing abnormal proteins that disrupt cell membranes

Aminoglycosides cause misreading at the 30S ribosome, incorporating wrong amino acids that create aberrant proteins which disrupt the cell membrane, causing bactericidal cell death.

Question 50: Which receptor does naloxone primarily antagonize to reverse opioid overdose?

Delta (δ) opioid receptor
Sigma (σ) receptor
Kappa (κ) opioid receptor
Mu (μ) opioid receptor (Correct answer)

Correct answer: Mu (μ) opioid receptor

Naloxone is a competitive antagonist primarily at mu-opioid receptors, rapidly reversing opioid-induced respiratory depression, sedation, and analgesia.

Question 51: A patient with severe combined immunodeficiency (SCID) has absent T and B cells. ADA deficiency is found. What metabolite accumulates and is toxic to lymphocytes?

Orotic acid
Deoxyadenosine triphosphate (dATP) (Correct answer)
Homocysteine
Methylmalonic acid

Correct answer: Deoxyadenosine triphosphate (dATP)

ADA deficiency causes accumulation of deoxyadenosine, which is phosphorylated to toxic dATP in lymphocytes, inhibiting ribonucleotide reductase and blocking DNA synthesis.

Question 52: Choose the correct answer

Heat stable toxins
P pili (Correct answer)
Type 1 pili

Correct answer: P pili

P pili (or Pap fimbriae) are a major virulence factor for uropathogenic *E. coli* (UPEC), which are the most common cause of urinary tract infections (UTIs). These fimbriae allow the bacteria to adhere specifically to galactosyl-alpha-1-4-galactose (Gal-Gal) receptors on uroepithelial cells in the urinary tract. This adherence is crucial for colonization and preventing the bacteria from being flushed out by urine flow.

Question 53: The juxtaglomerular apparatus (JGA) releases renin in response to which stimulus?

Increased angiotensin II
Decreased renal perfusion pressure (Correct answer)
Increased sodium delivery to the macula densa
Increased blood pressure

Correct answer: Decreased renal perfusion pressure

JGA granular cells release renin in response to decreased renal perfusion pressure (baroreceptor mechanism), decreased NaCl at the macula densa, and sympathetic stimulation (beta-1 receptors).

Question 54: Choose the correct answer

Complement (C6-C8) components (Correct answer)
Ciliary function
C1 esterase inhibitor

Correct answer: Complement (C6-C8) components

Deficiencies in the terminal complement components (C5-C9), particularly C6, C7, or C8, impair the formation of the membrane attack complex (MAC). The MAC is crucial for lysing Gram-negative bacteria, especially Neisseria species. Therefore, individuals with these deficiencies are at increased risk for recurrent Neisseria infections, such as meningococcal meningitis or gonococcal infections.

Question 55: Choose the correct answer

12 hours (Correct answer)
9 hours
18 hours

Correct answer: 12 hours

Without specific context, 12 hours is a common interval in medicine. It often represents the dosing frequency for medications taken twice daily (BID) to maintain therapeutic levels. It can also be relevant for monitoring certain physiological parameters or for the half-life of specific substances in the body.

Question 56: In the citric acid cycle, which reaction is catalyzed by isocitrate dehydrogenase?

Citrate → isocitrate
Succinate → fumarate
Isocitrate → alpha-ketoglutarate + CO2 (Correct answer)
Alpha-ketoglutarate → succinyl-CoA

Correct answer: Isocitrate → alpha-ketoglutarate + CO2

Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate, producing CO2 and NADH.

Question 57: A biopsy of a cardiac valve shows fibrinous vegetations on both sides of the leaflets in a patient with rheumatic fever. What is the pathological term for these vegetations?

Bacterial vegetations
Aschoff bodies (Correct answer)
Libman-Sacks endocarditis
Verrucous vegetations

Correct answer: Aschoff bodies

Aschoff bodies are pathognomonic granulomatous lesions of rheumatic fever, containing multinucleated giant cells (Aschoff cells) and Anitschkow cells in the myocardium.

Question 58: Choose the correct answer

Amitriptyline (Correct answer)
Diazepam
Malathion
Cimetidine
Propranolol

Correct answer: Amitriptyline

Amitriptyline is a tricyclic antidepressant (TCA) known for its potent anticholinergic, antihistaminic, and alpha-adrenergic blocking properties. It is commonly used for depression, neuropathic pain, and migraine prophylaxis. Its side effect profile includes sedation, dry mouth, constipation, and orthostatic hypotension due to these receptor interactions.

Question 59: Choose the correct answer

Superior pancreaticoduodenal artery and inferior pancreaticoduodenal artery (Correct answer)
Left gastroepiploic artery and right gastroepiploic artery
Proper hepatic artery and gastroduodenal artery
Left gastric artery and right gastric artery
Right colic artery and middle colic artery

Correct answer: Superior pancreaticoduodenal artery and inferior pancreaticoduodenal artery

The head of the pancreas and the duodenum receive their primary arterial supply from the superior and inferior pancreaticoduodenal arteries. The superior pancreaticoduodenal artery is a branch of the gastroduodenal artery, while the inferior pancreaticoduodenal artery branches from the superior mesenteric artery. These two arteries form an important anastomotic network, ensuring robust blood flow to this vital region.

Question 60: Which organism is an obligate intracellular bacterium that causes Q fever and is transmitted through aerosolized particles from cattle or sheep?

Rickettsia rickettsii
Coxiella burnetii (Correct answer)
Chlamydia pneumoniae
Mycoplasma pneumoniae

Correct answer: Coxiella burnetii

Coxiella burnetii causes Q fever, is extremely resistant to environmental conditions, and is transmitted via inhalation of contaminated aerosols from livestock; no arthropod vector required.

Question 61: Choose the correct answer

Staphylococcus aureus
Clostridium botulinum
Clostridium perfringens
Vibrio cholerae
EHEC (Enterohemorrhagic Escherichia coli)
Bacillus cereus (Correct answer)

Correct answer: Bacillus cereus

Bacillus cereus causes two types of food poisoning: an emetic form and a diarrheal form. The emetic form is typically associated with reheated rice and is caused by a preformed heat-stable toxin, leading to rapid onset (1-5 hours) of nausea and vomiting. The diarrheal form is associated with meat and vegetables and involves a heat-labile toxin.

Question 62: A patient with jaundice, hemolytic anemia, and splenomegaly has a G6PD deficiency. Which pathway is primarily affected?

Hexose monophosphate shunt (Correct answer)
Citric acid cycle
Glycolysis
Gluconeogenesis

Correct answer: Hexose monophosphate shunt

G6PD deficiency impairs the hexose monophosphate shunt, reducing NADPH and glutathione regeneration needed to protect RBCs from oxidative damage.

Question 63: The sinoatrial (SA) node receives its arterial supply from the SA nodal artery, which arises from which vessel in approximately 60% of individuals?

Posterior descending artery
Right coronary artery (Correct answer)
Left circumflex artery
Left anterior descending artery

Correct answer: Right coronary artery

In approximately 60% of individuals, the SA nodal artery arises from the right coronary artery; in the remaining ~40%, it arises from the left circumflex artery.

Question 64: Choose the correct answer

Aortic dissection (Correct answer)
Syphilitic aneurysm
Aortic valve stenosis

Correct answer: Aortic dissection

Aortic dissection is a life-threatening condition where a tear in the intima allows blood to enter the media, creating a false lumen. It typically presents with sudden onset of severe, 'tearing' or 'ripping' chest or back pain that may radiate. Risk factors include hypertension and connective tissue disorders like Marfan syndrome.

Question 65: Which enzyme is deficient in phenylketonuria (PKU)?

Tyrosinase
Phenylalanine hydroxylase (Correct answer)
Homogentisate oxidase
Fumarylacetoacetate hydrolase

Correct answer: Phenylalanine hydroxylase

PKU is caused by deficiency of phenylalanine hydroxylase, which converts phenylalanine to tyrosine.

Question 66: Which segment of the nephron is responsible for the countercurrent multiplication that generates the medullary osmotic gradient?

Loop of Henle (Correct answer)
Distal convoluted tubule
Collecting duct
Proximal convoluted tubule

Correct answer: Loop of Henle

The loop of Henle creates the medullary osmotic gradient through countercurrent multiplication: the descending limb is permeable to water (concentrating tubular fluid) while the ascending limb actively pumps NaCl out.

Question 67: A patient develops a Pancoast tumor at the apex of the right lung and presents with ptosis, miosis, and anhidrosis on the right side of the face. Which structure is most likely being compressed?

Right phrenic nerve
Brachial plexus upper trunk (C5-C6)
Right vagus nerve
Cervical sympathetic chain (Correct answer)

Correct answer: Cervical sympathetic chain

Pancoast tumors compress the cervical sympathetic chain, interrupting the sympathetic pathway and causing ipsilateral Horner's syndrome (ptosis, miosis, anhidrosis).

Question 68: A patient develops gynecomastia while taking spironolactone. What is the mechanism of this side effect?

Stimulation of estrogen receptors
Antagonism of androgen receptors (Correct answer)
Inhibition of CYP17A1
Inhibition of 5-alpha reductase

Correct answer: Antagonism of androgen receptors

Spironolactone non-selectively antagonizes androgen receptors in addition to mineralocorticoid receptors, causing gynecomastia and sexual dysfunction in males.

Question 69: The femoral sheath is divided into three compartments. Which structure occupies the most medial compartment (femoral canal)?

Lymphatics and loose connective tissue (Correct answer)
Femoral artery
Femoral nerve
Femoral vein

Correct answer: Lymphatics and loose connective tissue

The femoral canal is the most medial compartment of the femoral sheath and contains lymphatics and loose connective tissue, forming the site of potential femoral hernias.

Question 70: Choose the correct answer

Brucella canis
Clostridium tetani
Toxocara canis
Pasteurella multocida (Correct answer)
Bartonella henselae

Correct answer: Pasteurella multocida

Pasteurella multocida is a common bacterium found in the oral flora of cats and dogs. It is a frequent cause of infection following animal bites or scratches, leading to rapidly developing cellulitis, osteomyelitis, or septic arthritis. The infection typically presents with erythema, swelling, and pain at the bite site within 24 hours.

Question 71: Choose the correct answer

"Doctor, you must be wrong." (Correct answer)
"Damn you doctor, you should have caught this earlier!"
"I think it is time that I make a will and say good-bye to everyone."

Correct answer: "Doctor, you must be wrong."

This question likely refers to Kübler-Ross's stages of grief. 'Doctor, you must be wrong' represents the stage of denial, which is typically the first reaction to receiving a devastating diagnosis. In this stage, individuals struggle to accept the reality of their situation.

Question 72: A 55-year-old man presents with severe epigastric pain radiating to the back, nausea, and vomiting. Serum lipase is 5x the upper limit of normal. What are the two most common causes?

Trauma and ERCP
Hypertriglyceridemia and medications
Gallstones and alcohol (Correct answer)
NSAID use and hypercalcemia

Correct answer: Gallstones and alcohol

Gallstones (biliary pancreatitis) and alcohol abuse together account for approximately 80% of acute pancreatitis cases in the United States.

Question 73: Choose the correct answer

Chronic arthritis
Splenomegaly
Achalasia (Correct answer)

Correct answer: Achalasia

Achalasia is an esophageal motility disorder characterized by impaired relaxation of the lower esophageal sphincter (LES) and loss of peristalsis in the distal esophagus. This leads to difficulty swallowing (dysphagia) for both solids and liquids, regurgitation, and chest pain. The classic radiographic finding is a 'bird's beak' appearance of the distal esophagus.

Question 74: A patient develops a saddle-shaped nose deformity, perforation of the nasal septum, and granulomatous inflammation. cANCA is positive. What is the diagnosis?

Churg-Strauss syndrome
Granulomatosis with polyangiitis (Wegener's) (Correct answer)
Goodpasture syndrome
Polyarteritis nodosa

Correct answer: Granulomatosis with polyangiitis (Wegener's)

Granulomatosis with polyangiitis (formerly Wegener's) causes necrotizing granulomatous inflammation of the upper and lower respiratory tract and glomerulonephritis, with positive cANCA (PR3-ANCA).

Question 75: A CSF sample shows gram-negative diplococci. What is the most likely causative organism in an 18-year-old college student?

Streptococcus pneumoniae
Neisseria meningitidis (Correct answer)
Listeria monocytogenes
Haemophilus influenzae

Correct answer: Neisseria meningitidis

Neisseria meningitidis is the most common cause of bacterial meningitis in adolescents and young adults, particularly in close-contact settings like college dormitories.

Question 76: A 30-year-old woman has a breast mass that moves freely, is rubbery, and has well-defined borders. No lymph node involvement. What is the most likely diagnosis?

Ductal carcinoma in situ
Phyllodes tumor
Fibroadenoma (Correct answer)
Infiltrating ductal carcinoma

Correct answer: Fibroadenoma

Fibroadenoma is the most common benign breast tumor in young women, presenting as a well-circumscribed, rubbery, freely movable mass with no lymph node involvement.

Question 77: Which phase of the cardiac action potential in ventricular cardiomyocytes is responsible for the prolonged plateau and is mediated by L-type calcium channels?

Phase 4 (resting membrane potential)
Phase 0 (rapid depolarization)
Phase 3 (rapid repolarization)
Phase 2 (plateau phase) (Correct answer)

Correct answer: Phase 2 (plateau phase)

Phase 2 of the ventricular action potential is the plateau phase maintained by calcium influx through L-type (long-lasting) calcium channels, balanced by potassium efflux.

Question 78: Choose the correct answer

“Your mother was too hard to live with.”
“You’re the man of the house now.”
“I left your mother, I didn’t leave you.” (Correct answer)
“Big boys don’t cry.”
“I’ll see you in 2 weeks.”

Correct answer: “I left your mother, I didn’t leave you.”

When parents divorce, children often internalize the separation, feeling responsible or abandoned. Reassuring a child that the parent is not leaving them personally, but rather the other parent, helps to alleviate feelings of guilt and abandonment. This statement validates the child's feelings while clarifying the nature of the parental separation.

Question 79: During a thyroidectomy a surgeon must preserve all intrinsic laryngeal muscles. Which muscle is NOT innervated by the recurrent laryngeal nerve?

Posterior cricoarytenoid
Cricothyroid (Correct answer)
Thyroarytenoid
Lateral cricoarytenoid

Correct answer: Cricothyroid

The cricothyroid is the only intrinsic laryngeal muscle innervated by the external branch of the superior laryngeal nerve, not the recurrent laryngeal nerve.

Question 80: A patient taking lithium develops signs of toxicity including coarse tremor, confusion, and cardiac arrhythmias. What electrolyte abnormality predisposes to lithium toxicity?

Hyperkalemia
Hypernatremia
Hyponatremia (Correct answer)
Hypokalemia

Correct answer: Hyponatremia

Hyponatremia (from diuretics, low-sodium diet, or dehydration) causes the kidney to reabsorb lithium in place of sodium in the proximal tubule, increasing lithium levels and toxicity.

Question 81: Which type of necrosis is characteristic of tuberculosis granulomas?

Fat necrosis
Coagulative necrosis
Caseous necrosis (Correct answer)
Liquefactive necrosis

Correct answer: Caseous necrosis

Caseous necrosis is characteristic of granulomatous inflammation in TB, producing a cheese-like gross appearance due to incomplete digestion of cellular debris.

Question 82: A child is vaccinated with a live attenuated vaccine. Why is this vaccine contraindicated in immunocompromised patients?

The vaccine causes excessive antibody production
The attenuated virus can revert to virulence and cause disease in immunocompromised hosts (Correct answer)
Adjuvants in live vaccines cause systemic inflammation
The killed virus cannot stimulate immunity

Correct answer: The attenuated virus can revert to virulence and cause disease in immunocompromised hosts

Live attenuated vaccines can cause disease in immunocompromised patients because the attenuated organism may revert to virulence when the immune system cannot control even weakened pathogens.

Question 83: Which lipoprotein is primarily responsible for reverse cholesterol transport?

HDL (Correct answer)
LDL
Chylomicrons
VLDL

Correct answer: HDL

HDL mediates reverse cholesterol transport by collecting cholesterol from peripheral tissues and delivering it to the liver for excretion.

Question 84: A patient presents with weakness of finger abduction and adduction, loss of sensation over the medial 1.5 fingers and hypothenar eminence, and a 'claw hand' deformity affecting primarily the ring and little fingers. Which nerve is most likely injured?

Median nerve
Ulnar nerve (Correct answer)
Musculocutaneous nerve
Radial nerve

Correct answer: Ulnar nerve

The ulnar nerve innervates the interossei (finger abduction/adduction), hypothenar muscles, and provides sensation to the medial 1.5 fingers; injury causes claw hand due to loss of intrinsic muscle function.

Question 85: Choose the correct answer

Pituitary adenoma
Parathyroid adenoma
Pheochromocytoma (Correct answer)

Correct answer: Pheochromocytoma

Pheochromocytoma is a tumor of the adrenal medulla that secretes excessive amounts of catecholamines (epinephrine and norepinephrine). This leads to classic symptoms such as episodic hypertension, palpitations, headaches, and sweating. It is important to diagnose and treat pheochromocytoma due to the risk of hypertensive crises and cardiovascular complications.

Question 86: A patient with AIDS develops Pneumocystis jirovecii pneumonia. What prophylaxis should have been started when CD4 count fell below 200 cells/μL?

Dapsone
Azithromycin
TMP-SMX (trimethoprim-sulfamethoxazole) (Correct answer)
Fluconazole

Correct answer: TMP-SMX (trimethoprim-sulfamethoxazole)

TMP-SMX is the first-line prophylaxis for PCP when CD4 count falls below 200 cells/μL in HIV-infected patients.

Question 87: Choose the correct answer

Pons
Caudate nucleus (Correct answer)
Cerebellum

Correct answer: Caudate nucleus

Huntington's disease is a neurodegenerative disorder characterized by progressive motor, cognitive, and psychiatric symptoms. Pathologically, it is associated with selective atrophy of the caudate nucleus and putamen (components of the striatum). This neuronal loss, particularly in the caudate, leads to the characteristic choreiform movements seen in the disease.

Question 88: Choose the correct answer

Type I pneumocytes
Type II pneumocytes (Correct answer)
Bronchial respiratory epithelium

Correct answer: Type II pneumocytes

Type II pneumocytes are cuboidal cells found in the alveolar walls. Their primary functions include producing and secreting surfactant, which reduces surface tension in the alveoli and prevents their collapse. They also serve as progenitor cells, capable of proliferating and differentiating into Type I pneumocytes to repair damaged alveolar lining.

Question 89: A 25-year-old woman has amenorrhea, elevated prolactin, and bitemporal hemianopia. MRI shows a pituitary mass. What is the diagnosis?

Prolactinoma (Correct answer)
Acromegaly
Craniopharyngioma
Empty sella syndrome

Correct answer: Prolactinoma

Prolactinoma (most common pituitary adenoma) secretes excess prolactin causing galactorrhea, amenorrhea/infertility in women, and compresses the optic chiasm causing bitemporal hemianopia.

Question 90: A patient with type 1 diabetes presents with nausea, vomiting, Kussmaul breathing, and fruity breath. ABG shows pH 7.20, glucose 450 mg/dL, and urine ketones positive. What is the treatment priority?

Glucagon injection
Oral glucose
Immediate bicarbonate administration
IV fluid resuscitation and insulin infusion (Correct answer)

Correct answer: IV fluid resuscitation and insulin infusion

DKA treatment requires IV normal saline for volume resuscitation first, followed by insulin infusion (after K+ is ≥3.5 mEq/L) to suppress ketogenesis; potassium replacement is critical.

Question 91: Choose the correct answer

Amnestic disorder
Alzheimer disease (Correct answer)
Vascular dementia

Correct answer: Alzheimer disease

Alzheimer disease is the most common cause of dementia, characterized by progressive memory loss and cognitive decline. Pathologically, it involves the accumulation of amyloid plaques and neurofibrillary tangles (tau protein) in the brain, leading to neuronal dysfunction and death. It typically presents with insidious onset and gradual progression of symptoms.

Question 92: A patient has a Cushing reflex (hypertension, bradycardia, and irregular respirations). What does this indicate?

Neurogenic shock
Autonomic dysreflexia
Normal physiological response to exercise
Increased intracranial pressure threatening brainstem herniation (Correct answer)

Correct answer: Increased intracranial pressure threatening brainstem herniation

The Cushing reflex is a late sign of severely elevated ICP causing brainstem ischemia; the hypertension is a compensatory response to maintain cerebral perfusion against rising ICP.

Question 93: In the autonomic nervous system, which receptor mediates bradycardia when stimulated by the vagus nerve?

Beta-1 adrenergic receptor
Nicotinic receptor
Alpha-1 adrenergic receptor
Muscarinic M2 receptor (Correct answer)

Correct answer: Muscarinic M2 receptor

Vagal parasympathetic stimulation releases acetylcholine that binds M2 muscarinic receptors in the SA node, activating GIRK channels, hyperpolarizing the cell, and slowing heart rate.

Question 94: Which of the following structures correctly passes through the inguinal canal in males?

Round ligament of the uterus
Ilioinguinal nerve and spermatic cord (Correct answer)
Obturator nerve
Femoral nerve

Correct answer: Ilioinguinal nerve and spermatic cord

In males, the inguinal canal transmits the spermatic cord along with the ilioinguinal nerve; in females, the round ligament of the uterus passes through instead of the spermatic cord.

Question 95: A patient with recurrent Neisseria infections (meningococcal and gonococcal) has deficiency of which complement components?

C3
C4
C5-C9 (membrane attack complex) (Correct answer)
C1q, C1r, C1s

Correct answer: C5-C9 (membrane attack complex)

Deficiency in terminal complement components C5-C9 (membrane attack complex) prevents lysis of encapsulated gram-negative diplococci, predisposing to recurrent Neisseria infections.

Question 96: Which muscle is responsible for 'unlocking' the knee from full extension to allow flexion to begin?

Plantaris
Popliteus (Correct answer)
Gastrocnemius
Quadriceps femoris

Correct answer: Popliteus

The popliteus unlocks the knee from full extension by internally rotating the tibia on a fixed femur (screw-home reversal), allowing knee flexion to initiate.

Question 97: Which enzyme catalyzes the rate-limiting step of fatty acid synthesis?

Malonyl-CoA decarboxylase
Acetyl-CoA carboxylase (Correct answer)
Fatty acid synthase
HMG-CoA reductase

Correct answer: Acetyl-CoA carboxylase

Acetyl-CoA carboxylase catalyzes the conversion of acetyl-CoA to malonyl-CoA, which is the rate-limiting step of fatty acid synthesis.

Question 98: Choose the correct answer

sphingomyelin accumulation
vitamin A deficiency (Correct answer)
iron deficiency anemia

Correct answer: vitamin A deficiency

Vitamin A deficiency is a common cause of night blindness (nyctalopia) and xerophthalmia (dry eyes). Vitamin A is crucial for the formation of rhodopsin, a photopigment in the rod cells of the retina, which is essential for vision in low light conditions. Severe deficiency can lead to permanent blindness.

Question 99: Choose the correct answer

Risk of systemic thromboembolism (Correct answer)
Infectious etiology
Mitral valve stenosis

Correct answer: Risk of systemic thromboembolism

This question likely refers to atrial fibrillation or another condition causing stasis of blood in the heart. In conditions like atrial fibrillation, the irregular and often rapid contraction of the atria leads to blood stasis, particularly in the left atrial appendage. This stasis promotes clot formation, significantly increasing the risk of systemic thromboembolism, such as stroke.

Question 100: Which antiepileptic drug works by blocking voltage-gated sodium channels and is also used for neuropathic pain and bipolar disorder?

Carbamazepine (Correct answer)
Valproate
Phenobarbital
Ethosuximide

Correct answer: Carbamazepine

Carbamazepine blocks voltage-gated sodium channels and is used for partial seizures, trigeminal neuralgia, and bipolar disorder.

Question 101: Choose the correct answer

Plasma creatinine concentration
Creatinine production
Daily excretion of sodium
Creatinine clearance (Correct answer)
Renal excretion of creatinine

Correct answer: Creatinine clearance

Creatinine clearance is the most commonly used clinical measure to estimate the glomerular filtration rate (GFR), which reflects overall kidney function. It quantifies the volume of blood plasma cleared of creatinine per unit time. This provides a practical and relatively accurate assessment of how well the kidneys are filtering waste products from the blood.

Question 102: Which step in heme synthesis occurs in the mitochondria?

Formation of uroporphyrinogen III
Conversion of coproporphyrinogen to protoporphyrin
Conversion of porphobilinogen to hydroxymethylbilane
Formation of delta-aminolevulinic acid (ALA) from glycine and succinyl-CoA (Correct answer)

Correct answer: Formation of delta-aminolevulinic acid (ALA) from glycine and succinyl-CoA

The first step, formation of ALA from glycine and succinyl-CoA by ALA synthase, occurs in the mitochondrial matrix.

Question 103: Which gram-positive rod forms spores, is found in soil, and causes gas gangrene by producing a lecithinase (alpha-toxin)?

Clostridium tetani
Clostridium perfringens (Correct answer)
Clostridium botulinum
Bacillus anthracis

Correct answer: Clostridium perfringens

C. perfringens produces alpha-toxin (lecithinase/phospholipase C), which hydrolyzes membrane phospholipids, causing myonecrosis (gas gangrene) and hemolysis.

Question 104: Which pulmonary function test finding is characteristic of obstructive lung disease?

Increased TLC with decreased FVC
Decreased FEV1/FVC ratio (<0.70) (Correct answer)
Decreased DLCO only
Decreased FVC with normal FEV1/FVC ratio

Correct answer: Decreased FEV1/FVC ratio (<0.70)

Obstructive diseases (asthma, COPD, emphysema) show decreased FEV1/FVC ratio (<0.70) because airflow obstruction disproportionately reduces FEV1 more than FVC.

Question 105: Which change in pulmonary vascular resistance occurs during lung hyperinflation in emphysema?

Decreased pulmonary vascular resistance
No change in pulmonary vascular resistance
Increased pulmonary vascular resistance leading to cor pulmonale (Correct answer)
Increased pulmonary capillary wedge pressure

Correct answer: Increased pulmonary vascular resistance leading to cor pulmonale

Emphysema destroys alveolar walls and pulmonary capillaries, reducing the vascular bed and increasing resistance; hypoxic vasoconstriction further increases PVR, leading to right heart failure (cor pulmonale).

Question 106: Which gram-positive organism produces a toxin that causes toxic shock syndrome by acting as a superantigen?

Bacillus anthracis
Staphylococcus aureus (TSST-1) (Correct answer)
Clostridium perfringens
Streptococcus pyogenes (SPE)

Correct answer: Staphylococcus aureus (TSST-1)

S. aureus produces TSST-1, a superantigen that non-specifically activates up to 20% of T cells by cross-linking MHC II on APCs with TCR Vβ regions, causing massive cytokine release.

Question 107: Choose the correct answer

Delirium (Correct answer)
Dementia
Brief psychotic disorder

Correct answer: Delirium

Delirium is an acute, fluctuating disturbance in attention and cognition, often accompanied by altered consciousness. It is typically caused by an underlying medical condition, substance intoxication, or withdrawal. Key features include its sudden onset, fluctuating course, and often reversible nature once the underlying cause is addressed.

Question 108: Choose the correct answer

Pulmonary sequestration
Pulmonary immaturity
Congenital pulmonary cysts
Bronchogenic cysts
Posterior deviation of the tracheoesophageal septum (Correct answer)

Correct answer: Posterior deviation of the tracheoesophageal septum

Esophageal atresia with tracheoesophageal fistula (TEF) is a congenital anomaly resulting from incomplete separation of the foregut into the esophagus and trachea during embryonic development. This occurs due to abnormal posterior deviation of the tracheoesophageal septum, leading to a blind-ending esophagus and an abnormal connection between the trachea and esophagus.

Question 109: A patient presents with fever, right lower quadrant pain, rebound tenderness, and elevated WBC. CT shows fat stranding around a dilated appendix. What is the diagnosis?

Acute appendicitis (Correct answer)
Ovarian torsion
Mesenteric lymphadenitis
Crohn disease

Correct answer: Acute appendicitis

Acute appendicitis presents with RLQ pain (McBurney point), fever, leukocytosis, and CT showing periappendiceal fat stranding with appendiceal dilation (>6mm), requiring urgent appendectomy.

Question 110: Which hepatitis virus uses RNA-dependent RNA polymerase and requires co-infection with hepatitis B virus for replication?

Hepatitis A (HAV)
Hepatitis E (HEV)
Hepatitis C (HCV)
Hepatitis D (HDV) (Correct answer)

Correct answer: Hepatitis D (HDV)

Hepatitis D (delta virus) is a defective RNA virus that requires HBV surface antigen (HBsAg) for encapsidation, and can only infect HBV-positive patients (co- or superinfection).

Question 111: A patient with a Colles' fracture develops weakness of thumb opposition and loss of sensation over the lateral palm and first 3.5 fingers. Which nerve is most likely affected?

Radial nerve
Ulnar nerve
Musculocutaneous nerve
Median nerve (Correct answer)

Correct answer: Median nerve

The median nerve innervates the thenar muscles via its recurrent branch and provides sensory innervation to the lateral palm and first 3.5 fingers; it can be compressed in the carpal tunnel following a Colles' fracture.

Question 112: A 50-year-old man with a history of GERD presents with progressive dysphagia, first to solids then liquids, and weight loss. Endoscopy shows a mucosal irregularity. What is the most likely diagnosis?

Esophageal spasm
Esophageal carcinoma (Correct answer)
Zenker diverticulum
Achalasia

Correct answer: Esophageal carcinoma

Progressive dysphagia starting with solids then progressing to liquids combined with weight loss is the classic presentation of esophageal carcinoma; GERD is a risk factor for adenocarcinoma.

Question 113: Which amino acid is the primary gluconeogenic substrate released from skeletal muscle during fasting?

Glycine
Glutamine
Serine
Alanine (Correct answer)

Correct answer: Alanine

Alanine is the primary gluconeogenic amino acid exported from skeletal muscle to the liver, representing the glucose-alanine cycle.

Question 114: A patient develops hemolytic anemia, thrombocytopenia, and renal failure after a bloody diarrheal illness caused by E. coli O157:H7. What toxin causes this?

Heat-stable toxin (ST)
Hemolysin
Heat-labile toxin (LT)
Shiga toxin (Stx) (Correct answer)

Correct answer: Shiga toxin (Stx)

Shiga toxin (produced by EHEC O157:H7) inactivates the 60S ribosomal subunit by cleaving 28S rRNA, causing cell death in renal endothelium and leading to hemolytic uremic syndrome (HUS).

Question 115: A 55-year-old smoker presents with a central lung mass, hypercalcemia, and finger clubbing. Histology shows keratin pearls and intercellular bridges. What is the diagnosis?

Small cell carcinoma
Adenocarcinoma
Squamous cell carcinoma (Correct answer)
Large cell carcinoma

Correct answer: Squamous cell carcinoma

Squamous cell carcinoma arises centrally, produces PTHrP causing hypercalcemia, and shows keratin pearls with intercellular bridges on histology.

Question 116: Choose the correct answer

Superior segment of the right lower lobe (Correct answer)
Apical segment of the right upper lobe
Apical segment of the left upper lobe
Medial segment of the right middle lobe
Posterior basal segment of the left lower lobe

Correct answer: Superior segment of the right lower lobe

Aspiration pneumonia commonly affects the dependent lung segments due to gravity. In an upright or semi-recumbent position, the superior segment of the right lower lobe is the most common site for aspiration. If a patient is supine, the posterior segments of the upper lobes or the superior segments of the lower lobes are more likely to be affected.

Question 117: A patient with primary hyperaldosteronism (Conn syndrome) will have which electrolyte abnormality?

Hypokalemia and hypertension (Correct answer)
Hypernatremia and hypotension
Hyponatremia and hypotension
Hyperkalemia and hypertension

Correct answer: Hypokalemia and hypertension

Excess aldosterone promotes sodium retention and potassium excretion in the collecting duct, causing hypertension (volume expansion) and hypokalemia.

Question 118: Which vitamin is a cofactor for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase?

Niacin (B3)
Pyridoxine (B6)
Riboflavin (B2)
Thiamine (B1) (Correct answer)

Correct answer: Thiamine (B1)

Thiamine (B1) as thiamine pyrophosphate (TPP) is essential for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase reactions.

Question 119: A patient with SLE has a positive ANA and anti-dsDNA antibodies. Which type of hypersensitivity reaction mediates the kidney damage in lupus nephritis?

Type IV (cell-mediated)
Type III (immune complex-mediated) (Correct answer)
Type I (IgE-mediated)
Type II (antibody-mediated cytotoxicity)

Correct answer: Type III (immune complex-mediated)

Lupus nephritis is caused by deposition of immune complexes (dsDNA-anti-dsDNA) in the glomerular basement membrane, activating complement and causing type III hypersensitivity injury.

Question 120: A patient with cirrhosis develops ascites and portal hypertension. What type of hypertension is most commonly caused by cirrhosis?

Sinusoidal (intrahepatic) portal hypertension (Correct answer)
Presinusoidal portal hypertension
Budd-Chiari syndrome
Post-sinusoidal portal hypertension

Correct answer: Sinusoidal (intrahepatic) portal hypertension

Cirrhosis causes sinusoidal portal hypertension by increasing intrahepatic vascular resistance through fibrosis and regenerative nodule distortion of the sinusoidal architecture.

Question 121: In the urea cycle, which amino acid donates the second nitrogen atom incorporated into urea?

Glutamine
Aspartate (Correct answer)
Alanine
Glutamate

Correct answer: Aspartate

Aspartate donates the second nitrogen to form argininosuccinate in the urea cycle, ultimately contributing one nitrogen to urea.

Question 122: A patient with type 2 diabetes develops lactic acidosis. Which medication is most likely responsible?

Metformin (Correct answer)
Glipizide
Pioglitazone
Sitagliptin

Correct answer: Metformin

Metformin can cause lactic acidosis, particularly in patients with renal insufficiency, hepatic disease, or conditions causing tissue hypoperfusion.

Question 123: Choose the correct answer

Answer A
Answer C
Answer D (Correct answer)
Answer E
Answer B

Correct answer: Answer D

As the question and options are generic ('Answer A,' 'Answer B,' etc.), a specific medical explanation cannot be provided. In a typical USMLE question, 'Answer D' would represent a particular diagnosis, treatment, or physiological concept that is the correct response to a given clinical scenario or scientific query.

Question 124: A patient with type 2 diabetes presents with polydipsia, polyuria, and serum glucose of 650 mg/dL. Serum osmolality is 345 mOsm/kg. No ketones in urine. What is the diagnosis?

SIADH
Diabetic ketoacidosis (DKA)
Hyperosmolar hyperglycemic state (HHS) (Correct answer)
Diabetes insipidus

Correct answer: Hyperosmolar hyperglycemic state (HHS)

HHS is characterized by extreme hyperglycemia (>600 mg/dL), hyperosmolality (>320 mOsm/kg), and minimal ketosis, distinguishing it from DKA in type 2 diabetics with residual insulin.

Question 125: Which cranial nerve exits the skull through the foramen ovale?

Facial nerve (CN VII)
Ophthalmic nerve (CN V1)
Mandibular nerve (CN V3) (Correct answer)
Maxillary nerve (CN V2)

Correct answer: Mandibular nerve (CN V3)

The mandibular nerve (V3) exits through the foramen ovale; V1 exits through the superior orbital fissure, V2 through the foramen rotundum, and CN VII through the stylomastoid foramen.

Question 126: A patient with hypertension develops a sudden severe headache described as 'the worst headache of my life.' What is the most important diagnosis to rule out?

Subarachnoid hemorrhage (SAH) (Correct answer)
Migraine
Tension headache
Hypertensive urgency

Correct answer: Subarachnoid hemorrhage (SAH)

A thunderclap headache (worst headache of life, maximal at onset) is the classic presentation of subarachnoid hemorrhage, typically from a ruptured berry aneurysm, requiring urgent CT and LP.

Question 127: Which ACE inhibitor side effect is class-specific and due to bradykinin accumulation?

First-dose hypotension
Hyperkalemia
Angioedema
Dry cough (Correct answer)

Correct answer: Dry cough

ACE inhibitors prevent bradykinin degradation, causing dry cough in up to 15% of patients; ARBs do not inhibit bradykinin breakdown and lack this side effect.

Question 128: Choose the correct answer

Apex of left ventricle and anterior portion of septum
Lateral wall of the left ventricle only
Lateral left ventricular wall and posterior portion of the septum (Correct answer)

Correct answer: Lateral left ventricular wall and posterior portion of the septum

This question likely refers to the territory supplied by a specific coronary artery, most commonly the circumflex artery or its marginal branches. The left circumflex artery typically supplies the lateral wall of the left ventricle and can also supply the posterior portion of the septum in some individuals. Infarction in this region would indicate occlusion of this vessel.

Question 129: During exercise, which physiological change causes oxygen delivery to be increased to working muscles?

Rightward shift of the oxygen-hemoglobin dissociation curve (Correct answer)
Leftward shift of the oxygen-hemoglobin dissociation curve
Increased hemoglobin oxygen affinity
Decreased 2,3-DPG

Correct answer: Rightward shift of the oxygen-hemoglobin dissociation curve

Exercise increases CO2, temperature, H+, and 2,3-DPG in working muscles, shifting the O2-Hb dissociation curve rightward (Bohr effect), decreasing hemoglobin affinity and increasing O2 delivery.

Question 130: A 68-year-old man presents with progressive hoarseness. Imaging reveals a large aortic arch aneurysm. Which nerve is most likely compressed by this aneurysm?

Left phrenic nerve
Left superior laryngeal nerve
Right recurrent laryngeal nerve
Left recurrent laryngeal nerve (Correct answer)

Correct answer: Left recurrent laryngeal nerve

The left recurrent laryngeal nerve hooks under the aortic arch in the thorax, making it susceptible to compression from aortic arch aneurysms or mediastinal masses causing hoarseness.

Question 131: A patient on methotrexate develops mucositis and bone marrow suppression. Which agent is given to rescue normal cells?

Cyanocobalamin
Leucovorin (folinic acid) (Correct answer)
Pyridoxine
Folic acid

Correct answer: Leucovorin (folinic acid)

Leucovorin (folinic acid) bypasses DHFR inhibition by methotrexate, rescuing normal rapidly dividing cells from folate depletion.

Question 132: Maple syrup urine disease results from deficiency of which enzyme?

Propionyl-CoA carboxylase
Branched-chain alpha-ketoacid dehydrogenase (Correct answer)
Methylmalonyl-CoA mutase
Branched-chain aminotransferase

Correct answer: Branched-chain alpha-ketoacid dehydrogenase

Maple syrup urine disease is caused by deficiency of branched-chain alpha-ketoacid dehydrogenase, leading to accumulation of leucine, isoleucine, and valine.

Question 133: Which bacterial toxin is responsible for the classic symptoms of tetanus by blocking inhibitory neurotransmitter release?

Tetanospasmin (blocks GABA/glycine release at inhibitory interneurons) (Correct answer)
Botulinum toxin
Cholera toxin
Tetanolysin

Correct answer: Tetanospasmin (blocks GABA/glycine release at inhibitory interneurons)

Tetanospasmin cleaves synaptobrevin (VAMP) in inhibitory interneurons of the spinal cord, blocking GABA and glycine release, causing spastic paralysis.

Question 134: Choose the correct answer

Large shallow ulcers on the oral mucosa
Curdy white material overlying an erythematous base on the oral mucosa
Multiple small white spots on the buccal mucosa (Correct answer)
Cold sores on the lips
Adherent thin, whitish patch on gingiva

Correct answer: Multiple small white spots on the buccal mucosa

Multiple small white spots on the buccal mucosa, known as Koplik spots, are a pathognomonic sign of measles (rubeola). These spots typically appear 1-2 days before the characteristic maculopapular rash and are transient, disappearing as the rash progresses. They are considered an early diagnostic indicator of measles infection.

Question 135: Which beta-blocker is cardioselective and preferred in a patient with COPD requiring beta-blockade?

Labetalol
Propranolol
Carvedilol
Metoprolol (Correct answer)

Correct answer: Metoprolol

Metoprolol is a beta-1 selective blocker, making it safer in COPD patients by minimizing beta-2 bronchospasm compared to non-selective agents.

Question 136: Erythropoietin (EPO) is produced primarily by which cell type in response to hypoxia?

Peritubular fibroblasts of the renal cortex (Correct answer)
Juxtaglomerular cells
Glomerular mesangial cells
Proximal tubular epithelial cells

Correct answer: Peritubular fibroblasts of the renal cortex

EPO is produced by peritubular fibroblasts (interstitial cells) in the renal cortex in response to hypoxia via HIF-1alpha stabilization, stimulating red blood cell production in the bone marrow.

Question 137: Choose the correct answer

Sweating of the ipsilateral face
Proprioception from the ipsilateral leg
Vibratory sense from the ipsilateral arm (Correct answer)

Correct answer: Vibratory sense from the ipsilateral arm

This question likely refers to the sensory deficits associated with a specific lesion, possibly a spinal cord lesion affecting the dorsal columns. The dorsal columns carry vibratory sense and proprioception from the ipsilateral side of the body. Therefore, a lesion affecting the dorsal column at a cervical level would impair vibratory sense from the ipsilateral arm.

Question 138: Alkaptonuria is caused by a deficiency of which enzyme?

Fumarylacetoacetate hydrolase
Homogentisate oxidase (Correct answer)
Phenylalanine hydroxylase
Tyrosine aminotransferase

Correct answer: Homogentisate oxidase

Alkaptonuria results from homogentisate oxidase deficiency, causing accumulation of homogentisic acid, which causes dark urine and ochronosis.

Question 139: Starling forces govern fluid movement across capillary membranes. Which force promotes fluid reabsorption from the interstitium into capillaries?

Capillary hydrostatic pressure
Plasma oncotic pressure (COP) (Correct answer)
Interstitial oncotic pressure
Lymphatic obstruction

Correct answer: Plasma oncotic pressure (COP)

Plasma oncotic pressure (colloid osmotic pressure from plasma proteins, mainly albumin) draws fluid from the interstitium back into capillaries, counteracting capillary hydrostatic pressure.

Question 140: A positive tuberculin skin test (PPD) indicates which type of hypersensitivity reaction?

Type II (antibody-mediated cytotoxicity)
Type I (IgE-mediated)
Type IV (delayed-type hypersensitivity, T-cell mediated) (Correct answer)
Type III (immune complex)

Correct answer: Type IV (delayed-type hypersensitivity, T-cell mediated)

PPD reaction is a Type IV delayed-type hypersensitivity reaction mediated by sensitized memory T cells that recognize tuberculin antigens and recruit macrophages, peaking at 48-72 hours.

Question 141: Choose the correct answer

Cricothyroid (Correct answer)
Posterior cricoarytenoid
Thyroarytenoid
Vocalis
Lateral cricoarytenoid

Correct answer: Cricothyroid

The cricothyroid muscle is the primary tensor of the vocal cords, responsible for increasing their tension and thus raising the pitch of the voice. It is innervated by the external branch of the superior laryngeal nerve. Damage to this nerve or muscle can result in a monotone or hoarse voice.

Question 142: Choose the correct answer

Pseudomembranous colitis
Diverticulosis
Amebic colitis
Chronic appendicitis
Ulcerative colitis (Correct answer)

Correct answer: Ulcerative colitis

Ulcerative colitis is a chronic inflammatory condition affecting the colon, characterized by continuous inflammation that typically starts in the rectum and extends proximally. It involves only the mucosa and submucosa, leading to symptoms like bloody diarrhea, abdominal pain, and tenesmus. Crypt abscesses are a characteristic histological finding.

Question 143: Choose the correct answer

Raynaud disease
Sarcoidosis (Correct answer)
Scleroderma

Correct answer: Sarcoidosis

Sarcoidosis is a multisystem granulomatous disease of unknown etiology, characterized by the formation of non-caseating granulomas in various organs. It most commonly affects the lungs and lymph nodes, but can also involve the skin, eyes, liver, and heart. Clinical presentation varies widely depending on the organs involved.

Question 144: Which type of T helper cell is responsible for orchestrating the response against intracellular pathogens by activating macrophages through IFN-gamma?

Th1 (Correct answer)
Th17
Treg
Th2

Correct answer: Th1

Th1 cells produce IFN-gamma and TNF-beta, activating macrophages to destroy intracellular pathogens and driving cell-mediated immunity.

Question 145: A patient sustains a fibular neck fracture and subsequently develops foot drop with loss of sensation over the dorsum of the foot. Which nerve was most likely injured?

Sural nerve
Saphenous nerve
Common fibular (peroneal) nerve (Correct answer)
Tibial nerve

Correct answer: Common fibular (peroneal) nerve

The common fibular nerve winds superficially around the fibular neck, making it highly susceptible to injury; it innervates dorsiflexors and evertors of the foot, and its injury produces foot drop.

Question 146: When performing a lumbar puncture at L4-L5, which sequence of structures does the needle correctly traverse from superficial to deep?

Skin → ligamentum flavum → interspinous ligament → supraspinous ligament → epidural space → dura mater → subarachnoid space
Skin → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura mater → subdural space → arachnoid mater → subarachnoid space (Correct answer)
Skin → interspinous ligament → supraspinous ligament → epidural space → dura mater → arachnoid mater → subarachnoid space
Skin → supraspinous ligament → ligamentum flavum → interspinous ligament → epidural space → dura mater → subarachnoid space

Correct answer: Skin → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura mater → subdural space → arachnoid mater → subarachnoid space

A lumbar puncture needle traverses in order: skin, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura mater, subdural space, and arachnoid mater before entering the subarachnoid space.

Question 147: A patient presents with periorbital and peripheral edema, massive proteinuria (>3.5 g/day), hypoalbuminemia, and hyperlipidemia. What is this syndrome?

Acute glomerulonephritis
Chronic kidney disease
Nephritic syndrome
Nephrotic syndrome (Correct answer)

Correct answer: Nephrotic syndrome

Nephrotic syndrome is defined by the triad of massive proteinuria (>3.5 g/day), hypoalbuminemia, and edema, with hyperlipidemia and lipiduria as secondary features.

Question 148: Choose the correct answer

Sertoli-only syndrome
Testicular dysgenesis
Complete androgen resistance
17α-hydroxylase deficiency
5α-reductase deficiency (Correct answer)

Correct answer: 5α-reductase deficiency

5α-reductase deficiency is a disorder of sex development where testosterone cannot be converted to dihydrotestosterone (DHT). DHT is essential for the development of external male genitalia in utero. Individuals with this XY genetic makeup may present with ambiguous genitalia at birth, often virilizing at puberty due to increased testosterone.

Question 149: Which anticoagulant requires no laboratory monitoring and directly inhibits thrombin (Factor IIa)?

Dabigatran (Correct answer)
Fondaparinux
Rivaroxaban
Apixaban

Correct answer: Dabigatran

Dabigatran is a direct thrombin inhibitor that requires no routine INR monitoring and is reversed by idarucizumab.

Question 150: Choose the correct answer

Left atrium (Correct answer)
Right ventricle
Left ventricle

Correct answer: Left atrium

The left atrium receives oxygenated blood from the pulmonary veins and pumps it into the left ventricle. In mitral stenosis, the mitral valve opening is narrowed, impeding blood flow from the left atrium to the left ventricle. This leads to increased pressure and dilation of the left atrium, which can cause symptoms like dyspnea and predispose to atrial fibrillation.

Question 151: Which hormone directly stimulates osteoclast activity and increases serum calcium and phosphate reabsorption in the kidney?

1,25-dihydroxyvitamin D3
Calcitonin
PTH (increases calcium, decreases phosphate reabsorption) (Correct answer)
PTHrP

Correct answer: PTH (increases calcium, decreases phosphate reabsorption)

PTH activates osteoclasts (indirectly via osteoblasts), increases renal calcium reabsorption, decreases phosphate reabsorption, and stimulates 1-alpha hydroxylase, increasing calcitriol synthesis.

Question 152: Choose the correct answer

Vascular resistance in the lower body (Correct answer)
Vascular resistance in the upper body
Blood pressure in the upper body
Blood flow in the upper body
Blood flow in the lower body

Correct answer: Vascular resistance in the lower body

Coarctation of the aorta is a congenital narrowing of the aorta, typically distal to the left subclavian artery. This narrowing significantly increases the vascular resistance in the lower body, leading to decreased blood flow and pressure in the lower extremities while increasing blood pressure in the upper extremities. The body compensates by developing collateral circulation.

Question 153: A patient sustains a stab wound to the posterior triangle of the neck and subsequently develops shoulder droop with difficulty elevating the arm above 90 degrees. Which nerve was most likely injured?

Dorsal scapular nerve
Phrenic nerve
Long thoracic nerve
Accessory nerve (CN XI) (Correct answer)

Correct answer: Accessory nerve (CN XI)

The accessory nerve (CN XI) traverses the posterior triangle of the neck and innervates the trapezius; its injury causes shoulder droop and impairs full arm elevation.

Question 154: A 70-year-old woman with osteoporosis has a compression fracture. DEXA scan shows T-score of -2.8. Which medication reduces fracture risk by inhibiting osteoclast function?

Teriparatide
Raloxifene
Denosumab
Alendronate (bisphosphonate) (Correct answer)

Correct answer: Alendronate (bisphosphonate)

Bisphosphonates like alendronate are first-line therapy for osteoporosis; they bind to hydroxyapatite and inhibit osteoclast-mediated bone resorption, reducing fracture risk by approximately 50%.

Question 155: Choose the correct answer

Diffuse form of scleroderma
Drug-induced lupus (Correct answer)
Sjögren syndrome

Correct answer: Drug-induced lupus

Drug-induced lupus erythematosus (DILE) is a lupus-like syndrome caused by certain medications, such as procainamide, hydralazine, and isoniazid. It typically presents with arthralgias, myalgias, and serositis, but rarely involves renal or CNS manifestations. A key laboratory finding is the presence of anti-histone antibodies, and symptoms usually resolve upon discontinuation of the offending drug.

Question 156: A patient taking isoniazid for TB develops peripheral neuropathy. Which vitamin supplementation prevents this?

Pyridoxine (B6) (Correct answer)
Thiamine (B1)
Niacin (B3)
Cyanocobalamin (B12)

Correct answer: Pyridoxine (B6)

Isoniazid competitively inhibits pyridoxal phosphate (active B6), causing peripheral neuropathy; B6 supplementation is given prophylactically.

Question 157: Choose the correct answer

Radial nerve
Brachial artery (Correct answer)
Profunda brachii artery
Ulnar nerve
Musculocutaneous nerve

Correct answer: Brachial artery

The brachial artery is the main artery of the upper arm, a continuation of the axillary artery. It is a crucial anatomical landmark for various medical procedures, including blood pressure measurement and arterial access. Its significant role in supplying blood to the forearm and hand makes it a frequently tested structure in anatomy and clinical contexts.

Question 158: Choose the correct answer

Myasthenia gravis
Idiopathic thrombocytopenic purpura (Correct answer)
Graves disease

Correct answer: Idiopathic thrombocytopenic purpura

Idiopathic thrombocytopenic purpura (ITP), now often called immune thrombocytopenia, is an autoimmune disorder characterized by a low platelet count (thrombocytopenia). It occurs when the immune system mistakenly produces antibodies that attack and destroy the body's own platelets. This leads to increased platelet destruction in the spleen and reduced platelet production, resulting in symptoms like easy bruising, petechiae, and bleeding.

Question 159: A child presents with megaloblastic anemia and homocystinuria but normal methylmalonic acid levels. Which vitamin deficiency is most likely?

Vitamin B6
Folate (Correct answer)
Vitamin B2
Vitamin B12

Correct answer: Folate

Folate deficiency causes megaloblastic anemia and elevated homocysteine but does not cause methylmalonic aciduria, distinguishing it from B12 deficiency.

Question 160: Choose the correct answer

Hypoglossal
Trigeminal
Vagus (Correct answer)

Correct answer: Vagus

The vagus nerve (Cranial Nerve X) is a mixed nerve with extensive parasympathetic innervation to the thoracic and abdominal viscera. It plays a crucial role in regulating heart rate, digestion, and respiratory function. Its motor component innervates muscles of the pharynx and larynx, important for swallowing and speech.

Question 161: Insulin promotes glucose uptake in muscle and adipose tissue by stimulating translocation of which glucose transporter?

GLUT2
GLUT1
GLUT4 (Correct answer)
GLUT3

Correct answer: GLUT4

Insulin stimulates translocation of GLUT4-containing vesicles to the plasma membrane in muscle and adipose tissue, increasing glucose uptake via PI3K-Akt signaling.

Question 162: The Frank-Starling mechanism states that increased preload leads to increased stroke volume. What is the molecular basis for this?

Increased overlap of actin and myosin leads to more cross-bridge formation (Correct answer)
Decreased afterload
Increased calcium influx
Increased heart rate

Correct answer: Increased overlap of actin and myosin leads to more cross-bridge formation

Stretching myocardial fibers (increased preload) increases actin-myosin overlap and sensitizes troponin C to calcium, increasing cross-bridge formation and contractile force.

Question 163: A patient develops torsades de pointes after starting a new antiarrhythmic. Which drug is most likely responsible?

Lidocaine
Sotalol (Correct answer)
Amiodarone
Adenosine

Correct answer: Sotalol

Sotalol (Class III) prolongs the QT interval by blocking K+ channels, predisposing to torsades de pointes, especially in patients with bradycardia or hypokalemia.

Question 164: Choose the correct answer

Serum alkaline phosphatase (Correct answer)
Urinary hydroxyproline
Prostatic acid phosphatase
Tartrate-resistant acid phosphatase
Prostate-specific antigen

Correct answer: Serum alkaline phosphatase

Serum alkaline phosphatase (ALP) is an enzyme found in various tissues, including bone, liver, and placenta. Elevated ALP levels, particularly the bone isoenzyme, are a sensitive indicator of increased osteoblastic activity. This makes it a useful marker for conditions involving increased bone turnover, such as metastatic bone disease, Paget's disease, and osteomalacia.

Question 165: Choose the correct answer

AIDS
Selective IgA deficiency (Correct answer)
Wiskott-Aldrich syndrome
C1 esterase inhibitor deficiency
DiGeorge syndrome

Correct answer: Selective IgA deficiency

Selective IgA deficiency is the most common primary immunodeficiency, characterized by low or absent serum IgA levels with normal levels of other immunoglobulins. Patients are often asymptomatic but may experience recurrent sinopulmonary and gastrointestinal infections, as IgA is crucial for mucosal immunity. Anaphylactic reactions to IgA-containing blood products can also occur.

Question 166: A patient has syndrome of inappropriate antidiuretic hormone (SIADH). What is the expected urine osmolality?

Isotonic urine
High urine osmolality (>100 mOsm/kg) (Correct answer)
Variable depending on fluid intake
Low urine osmolality (<50 mOsm/kg)

Correct answer: High urine osmolality (>100 mOsm/kg)

In SIADH, excess ADH causes inappropriate water retention and concentrated urine (>100 mOsm/kg, often >300 mOsm/kg) despite low serum osmolality.

Question 167: Which mechanism allows bacteria to acquire antibiotic resistance from the environment by taking up naked DNA?

Transduction
Vertical transmission
Conjugation
Transformation (Correct answer)

Correct answer: Transformation

Transformation is the uptake of naked DNA from the environment by naturally competent bacteria, allowing acquisition of resistance genes from dead bacteria.

Question 168: Choose the correct answer

Occipital sinus
Straight sinus
Cavernous sinus (Correct answer)
Superior petrosal sinus
Sigmoid sinus

Correct answer: Cavernous sinus

The cavernous sinus is a dural venous sinus located on either side of the sella turcica. It is clinically significant due to its close proximity to several cranial nerves (III, IV, V1, V2, VI) and the internal carotid artery. Infections or thromboses in this sinus can lead to severe neurological deficits, including ophthalmoplegia and facial sensory loss.

Question 169: A patient with chronic myeloid leukemia (CML) has the Philadelphia chromosome. Which genetic translocation produces this?

t(9;22) (Correct answer)
t(8;14)
t(15;17)
t(14;18)

Correct answer: t(9;22)

CML is characterized by t(9;22), creating the BCR-ABL fusion gene on the Philadelphia chromosome, which encodes a constitutively active tyrosine kinase.

Question 170: A patient on a high-fat diet has elevated chylomicrons and VLDL. Which enzyme is responsible for clearing chylomicrons from the blood?

Lipoprotein lipase (Correct answer)
Hepatic lipase
LCAT
Hormone-sensitive lipase

Correct answer: Lipoprotein lipase

Lipoprotein lipase on capillary endothelium hydrolyzes triglycerides in chylomicrons and VLDL, releasing fatty acids for peripheral uptake.

Question 171: Choose the correct answer

Streptococcus pyogenes
Enterococcus faecalis
Staphylococcus aureus
Staphylococcus epidermidis (Correct answer)
Streptococcus agalactiae

Correct answer: Staphylococcus epidermidis

Staphylococcus epidermidis is a common component of normal skin flora and is a frequent cause of infections associated with indwelling medical devices like prosthetic joints, catheters, and heart valves. It produces a biofilm that allows it to adhere to these surfaces and resist antibiotics. Unlike S. aureus, it is coagulase-negative.

Question 172: Choose the correct answer

Aspirin
Probenecid
Colchicine (Correct answer)

Correct answer: Colchicine

Colchicine is an anti-inflammatory agent primarily used for the acute treatment and prophylaxis of gout. It works by inhibiting microtubule polymerization, thereby disrupting neutrophil migration and activity in inflamed joints. This reduces the inflammatory response to uric acid crystals.

Question 173: Choose the correct answer

Rathke’s pouch (Correct answer)
Posterior pituitary gland
Pineal gland

Correct answer: Rathke’s pouch

Rathke's pouch is an ectodermal invagination from the roof of the primitive oral cavity that gives rise to the anterior pituitary gland (adenohypophysis). Craniopharyngiomas are tumors that arise from remnants of Rathke's pouch. These tumors are typically suprasellar and can cause symptoms due to compression of surrounding structures like the optic chiasm or hypothalamus, often leading to endocrine dysfunction.

Question 174: Choose the correct answer

Ectoderm
Neural crest (Correct answer)
Splanchnic mesoderm
Neural ectoderm
Endoderm

Correct answer: Neural crest

Neural crest cells are multipotent migratory cells that originate from the neural folds during embryonic development. They contribute to a wide array of tissues and cell types throughout the body, including the peripheral nervous system, melanocytes, adrenal medulla, and many craniofacial structures. Their extensive migratory capacity makes them crucial for diverse developmental processes.

Question 175: Coagulative necrosis is characteristic of infarction in most organs EXCEPT which organ?

Brain (Correct answer)
Spleen
Kidney
Heart

Correct answer: Brain

Brain infarction results in liquefactive necrosis due to the high water content and abundance of proteolytic enzymes; all other organs typically show coagulative necrosis after infarction.

Question 176: Choose the correct answer

Isospora belli (Correct answer)
Entamoeba histolytica
Diphyllobothrium latum
Microsporidia
Giardia lamblia

Correct answer: Isospora belli

Isospora belli (now Cystoisospora belli) is an intestinal parasite known to cause isosporiasis, a diarrheal illness. It is particularly associated with chronic, watery diarrhea in immunocompromised individuals, such as those with HIV/AIDS. Diagnosis involves identifying oocysts in stool, and treatment typically uses trimethoprim-sulfamethoxazole.

Question 177: Choose the correct answer

S-100
PSA
CA-125 (Correct answer)
Bombesin
Alpha-fetoprotein

Correct answer: CA-125

CA-125 (Cancer Antigen 125) is a tumor marker primarily used in the management of ovarian cancer. Elevated levels are often found in women with epithelial ovarian cancer, and it is utilized for monitoring treatment response and detecting recurrence. While not specific enough for definitive diagnosis alone, it is a valuable tool in conjunction with imaging and clinical evaluation.

Question 178: Which proto-oncogene, when amplified, is associated with poor prognosis in neuroblastoma?

N-MYC (Correct answer)
HER2/neu
RET
BCR-ABL

Correct answer: N-MYC

N-MYC (MYCN) amplification in neuroblastoma is associated with rapid tumor progression and poor prognosis, classified as Stage 4S in infants.

Question 179: Which cell type is most prominent in the early neutrophilic phase of acute inflammation?

Neutrophils (Correct answer)
Plasma cells
Macrophages
Lymphocytes

Correct answer: Neutrophils

Neutrophils are the first responders in acute inflammation, arriving within hours via chemotaxis and phagocytosing bacteria and debris.

Question 180: A patient with myasthenia gravis is treated with pyridostigmine. What is its mechanism of action?

Inhibition of choline acetyltransferase
Reversible inhibition of acetylcholinesterase (Correct answer)
Stimulation of nicotinic acetylcholine receptors
Irreversible inhibition of acetylcholinesterase

Correct answer: Reversible inhibition of acetylcholinesterase

Pyridostigmine reversibly inhibits acetylcholinesterase, increasing acetylcholine availability at the neuromuscular junction to compensate for autoimmune receptor loss.

Question 181: Choose the correct answer

Meningioma
Glioblastoma multiforme
Metastatic carcinoma (Correct answer)
Ependymoma
Astrocytoma

Correct answer: Metastatic carcinoma

Metastatic carcinoma is the most common malignant brain tumor in adults, often presenting as multiple lesions at the gray-white matter junction. The primary tumors frequently originate from the lung, breast, kidney, colon, or melanoma. These lesions typically have a well-demarcated appearance with surrounding edema on imaging.

Question 182: A patient with heart failure is given furosemide. Which electrolyte disturbance requires monitoring?

Hyponatremia
Hypercalcemia
Hyperkalemia
Hypokalemia (Correct answer)

Correct answer: Hypokalemia

Loop diuretics like furosemide cause urinary potassium wasting by increasing tubular flow to the collecting duct where K+ is secreted, leading to hypokalemia.

Question 183: Which glycogen storage disease presents with hypoglycemia, lactic acidosis, and hepatomegaly due to glucose-6-phosphatase deficiency?

Pompe disease (Type II)
Von Gierke disease (Type I) (Correct answer)
McArdle disease (Type V)
Cori disease (Type III)

Correct answer: Von Gierke disease (Type I)

Von Gierke disease (Type Ia) results from glucose-6-phosphatase deficiency, preventing glucose release from the liver and causing hypoglycemia with lactic acidosis.

Question 184: As the ureter descends into the pelvis, it crosses posterior to which structure at the pelvic brim?

Internal iliac artery
Common iliac artery bifurcation (Correct answer)
Femoral artery
External iliac artery

Correct answer: Common iliac artery bifurcation

The ureter crosses posterior to the bifurcation of the common iliac artery at the pelvic brim, an important landmark during pelvic and urologic surgery.

Question 185: Choose the correct answer

Rupture of the left ventricular wall (Correct answer)
Rupture of papillary muscle
Fatal arrhythmia

Correct answer: Rupture of the left ventricular wall

Rupture of the left ventricular wall is a catastrophic mechanical complication of acute myocardial infarction (MI), typically occurring 3-7 days post-MI. It involves necrosis and weakening of the infarcted myocardial tissue, leading to a tear in the ventricular wall. This results in hemopericardium and cardiac tamponade, which is rapidly fatal due to the heart's inability to pump blood effectively.

Question 186: A patient with chronic kidney disease has metabolic acidosis, hyperphosphatemia, and hypocalcemia. What secondary complication develops due to low 1,25-dihydroxyvitamin D3?

Hypoparathyroidism
Secondary hyperparathyroidism (Correct answer)
Pseudohypoparathyroidism
Primary hyperparathyroidism

Correct answer: Secondary hyperparathyroidism

CKD impairs 1-alpha hydroxylase (kidney), reducing 1,25-OH vitamin D3 production, causing hypocalcemia and secondary hyperparathyroidism with elevated PTH and renal osteodystrophy.

Question 187: Which mutation in p53 leads to a dominant negative effect, where the mutant protein inhibits normal p53 function?

Deletion of the entire p53 gene
Nonsense mutation causing truncation
Frameshift mutation
Missense mutation causing conformational change (Correct answer)

Correct answer: Missense mutation causing conformational change

Missense mutations in p53 produce a full-length but structurally abnormal protein that can complex with normal p53 tetramers, dominant-negatively impairing tumor suppression.

Question 188: Which of the following best describes the drainage territory of the right lymphatic duct?

Right side of the head, neck, thorax, and right upper limb (Correct answer)
All structures below the diaphragm on the right
Right lower limb and right side of the abdomen
Entire right side of the body

Correct answer: Right side of the head, neck, thorax, and right upper limb

The right lymphatic duct drains the right side of the head, neck, and thorax plus the right upper limb, emptying into the junction of the right internal jugular and subclavian veins.

Question 189: A 65-year-old man presents with chest pain radiating to the left arm, diaphoresis, and ST elevation in leads II, III, and aVF. Which coronary artery is most likely occluded?

Right coronary artery (RCA) (Correct answer)
Left circumflex artery (LCx)
Left anterior descending artery (LAD)
Left main coronary artery

Correct answer: Right coronary artery (RCA)

ST elevation in leads II, III, and aVF indicates inferior STEMI, which is most commonly caused by occlusion of the right coronary artery (RCA) that supplies the inferior wall.

United States Medical Licensing Examination (USMLE) Step 1

The USMLE Step 1 assesses a medical student's ability to apply basic science concepts to clinical situations. It is a critical step for medical licensure in the United States.

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