Microbiology Test 1 — Questions and Answers

Question 1: A 45-year-old woman comes to the physician because of progressive facial swelling and pain during the past week. She has a 10-year history of poorly controlled type 1 diabetes mellitus. Physical examination shows ecchymoses over the left orbital and periorbital regions with proptosis. There is a necrotic lesion with a black eschar in the left naris. Findings on microscopic examination of material from the lesion include broad, irregularly shaped, nonseptate hyphae with branches at right angles. Which of the following is the most likely cause of the nasal lesion?

Histoplasma capsulatum
Rhizopus oryzae (Correct answer)
Sporothrix schenckii
Torulopsis glabrata
Trichophyton rubrum

Correct answer: Rhizopus oryzae

The patient's presentation of rhinocerebral mucormycosis, characterized by facial swelling, proptosis, and a necrotic lesion with a black eschar in the naris, is highly suggestive of a Zygomycetes infection. Her poorly controlled type 1 diabetes is a significant risk factor for this aggressive fungal infection. The microscopic description of broad, irregularly shaped, nonseptate hyphae with branches at right angles is pathognomonic for Mucorales, which includes Rhizopus oryzae, the most common cause of mucormycosis.

Question 2: A 21-year-old woman who is a college student is brought to the emergency department 2 hours after the onset of fever, chills, severe headache, and confusion. Her temperature is 39°C (102.2°F), respirations are 16/min, and blood pressure is 100/60 mm Hg. Physical examination shows numerous petechial lesions over the upper and lower extremities. There is resistance to neck flexion. Analysis of cerebrospinal fluid shows numerous leukocytes and gram-negative diplococci. Administration of which of the following vaccines is most likely to have prevented this patient’s condition?

Haemophilus influenzae type b vaccine
Meningococcal conjugate vaccine, 4-valent (Correct answer)
Pneumococcal conjugate vaccine, 7-valent
Pneumococcal polysaccharide vaccine, 23-valent
Varicella vaccine

Correct answer: Meningococcal conjugate vaccine, 4-valent

The patient's symptoms of fever, severe headache, confusion, petechial lesions, and nuchal rigidity are classic for bacterial meningitis, specifically meningococcal meningitis given the petechiae. The cerebrospinal fluid analysis confirming numerous leukocytes and gram-negative diplococci points to Neisseria meningitidis. The meningococcal conjugate vaccine (MCV4) protects against the most common serogroups (A, C, W, Y) causing invasive disease and is recommended for college students due to their increased risk in close-quarter living environments.

Question 3: A sexually active 37-year-old woman comes to the physician because of a 2-day history of pain in the area of her genitals. Pelvic examination shows shallow, small, extremely tender ulcers with red bases in the vulvar and vaginal regions. A Tzanck smear shows the presence of multinucleated giant cells. Which of the following infectious agents is the most likely cause of these findings?

Candida albicans
Chlamydia trachomatis
Herpes simplex virus (Correct answer)
Treponema pallidum
Trichomonas vaginalis

Correct answer: Herpes simplex virus

The presence of shallow, small, extremely tender ulcers with red bases in the vulvar and vaginal regions is highly characteristic of genital herpes. The finding of multinucleated giant cells on a Tzanck smear is a classic cytological hallmark of herpes simplex virus (HSV) infection, confirming the viral etiology. HSV causes painful vesicular lesions that rupture to form ulcers, and the Tzanck smear helps differentiate it from other causes of genital ulcers.

Question 4: During an experimental study, an investigator develops a new member of the class of non-nucleoside reverse transcriptase inhibitors. This new drug most likely inhibits HIV infection through which of the following mechanisms?

Binding to reverse transcriptase (Correct answer)
Binding to RNase H
Blockade of integration
Incorporation into viral DNA
Phosphorylation of nucleoside trisphosphates

Correct answer: Binding to reverse transcriptase

Non-nucleoside reverse transcriptase inhibitors (NNRTIs) are a class of antiretroviral drugs specifically designed to inhibit HIV infection by binding directly to the reverse transcriptase enzyme. This binding induces a conformational change in the enzyme, thereby preventing it from converting viral RNA into DNA. This mechanism is distinct from nucleoside reverse transcriptase inhibitors (NRTIs), which are incorporated into the viral DNA chain, and effectively halts the HIV replication cycle.

Question 5: A 35-year-old woman delivers a newborn who develops meningitis. The mother has a streptococcus isolated from her vagina. The organism agglutinates with antiserum directed against type B surface carbohydrate. The virulence of this organism is related to a bacterial constituent that interferes with which of the following host phagocyte functions?

Aggregation
Chemotaxis
Ingestion (Correct answer)
Intracellular killing
Pseudopod formation

Correct answer: Ingestion

Group B Streptococcus (GBS), or Streptococcus agalactiae, is a major cause of neonatal meningitis and sepsis. Its primary virulence factor is its polysaccharide capsule, particularly the type B capsule mentioned. This capsule interferes with the host's immune response by preventing the ingestion (phagocytosis) of the bacteria by macrophages and neutrophils. By evading phagocytosis, GBS can survive and proliferate in the host, leading to invasive infections.

Question 6: A 33-year-old woman contracts malaria while on a 3-month business trip to a Central American country. She is treated with a full course of chloroquine and recovers uneventfully. Four months after returning to the USA, she has another febrile illness that resembles malaria. A peripheral blood smear shows ring forms in her erythrocytes. Which of the following species of Plasmodium is most likely to have caused the second febrile illness?

P. falciparum
P. knowlesi
P. malariae
P. vivax (Correct answer)

Correct answer: P. vivax

Plasmodium vivax (and P. ovale) are unique among malaria species in their ability to form dormant liver stages called hypnozoites. These hypnozoites are not eradicated by chloroquine, which only targets the erythrocytic (blood) stages of the parasite. Therefore, after successful treatment of the initial infection, hypnozoites can reactivate months later, causing a relapse of malaria, which perfectly explains the patient's second febrile illness with ring forms in erythrocytes.

Question 7: Three weeks after traveling to California to study desert flowers, a 33-year-old man develops fever, chest pain, and muscle soreness. Two days later, red, tender nodules appear on the shins, and the right ankle is tender and painful. An x-ray of the chest shows a left pleural effusion. Which of the following is the most likely diagnosis?

Blastomycosis
Coccidioidomycosis (Correct answer)
Histoplasmosis
Mycobacterium marinum infection
Mycoplasma pneumoniae infection

Correct answer: Coccidioidomycosis

The patient's travel history to California, an endemic region for Coccidioides immitis, combined with symptoms of fever, chest pain, muscle soreness, and the characteristic skin lesions of erythema nodosum (red, tender nodules on the shins), strongly points to coccidioidomycosis. This fungal infection, also known as 'desert fever,' often presents with respiratory symptoms and extrapulmonary manifestations like arthralgias and erythema nodosum, consistent with the patient's findings.

Question 8: At a banquet, the menu includes fried chicken, home-fried potatoes, peas, chocolate eclairs, and coffee. Within 2 hours, most of the diners become violently ill, with nausea, vomiting, abdominal pain, and diarrhea. Analysis of the contaminated food is most likely to yield large numbers of which of the following organisms?

Enterococcus faecalis
Escherichia coli
Proteus mirabilis
Salmonella typhimurium
Staphylococcus aureus (Correct answer)

Correct answer: Staphylococcus aureus

The rapid onset (within 2 hours) of severe nausea, vomiting, abdominal pain, and diarrhea after consuming food is highly characteristic of food poisoning caused by preformed bacterial toxins. Staphylococcus aureus produces heat-stable enterotoxins that, when ingested, quickly trigger these gastrointestinal symptoms. This type of food poisoning often occurs when foods, particularly those requiring handling like fried chicken or pastries, are left at room temperature, allowing the bacteria to grow and produce toxins.

Question 9: A 35-year-old woman is admitted to the hospital because of fever and dry cough for 3 days. An x-ray of the chest shows abnormal findings. Laboratory tests show cold agglutinating antibodies in serum. Results of routine cultures of blood and sputum are negative. Which of the following is the most likely pathogen?

Influenza virus
Mycobacterium tuberculosis
Mycoplasma pneumoniae (Correct answer)
Staphylococcus aureus
Streptococcus pneumoniae

Correct answer: Mycoplasma pneumoniae

The patient's presentation of a dry cough, fever, abnormal chest x-ray, and especially the presence of cold agglutinating antibodies in serum, strongly suggests Mycoplasma pneumoniae infection. Mycoplasma pneumoniae is a common cause of 'walking pneumonia,' characterized by a persistent dry cough and often occurring in younger adults. Routine bacterial cultures are typically negative because Mycoplasma lacks a cell wall and requires special media for growth, and cold agglutinins are a common, though non-specific, finding in these infections.

Question 10: A mutation that causes the loss of the 3´, 5´ exonuclease activity of DNA-dependent DNA polymerase is most likely to also cause Escherichia coli to have problems with which cellular process?

Replacing misincorporated bases (Correct answer)
Segregating sister chromosomes
Supercoiling of DNA
Synthesizing an RNA primer
Transferring DNA strands during conjugation

Correct answer: Replacing misincorporated bases

The 3´, 5´ exonuclease activity of DNA-dependent DNA polymerase is essential for its proofreading function during DNA replication. This activity allows the polymerase to detect and remove incorrectly incorporated nucleotides immediately after they are added to the growing DNA strand. Therefore, a mutation causing the loss of this exonuclease activity would significantly impair the cell's ability to replace misincorporated bases, leading to a much higher rate of mutations and genomic instability.

Question 11: During an experiment, an investigator gently abrades the skin from the flank of a mouse, creating a 1 × 2-cm skin window. A glass coverslip is then placed over the area so that cells attracted to the site attach to the coverslip for assessment. Two hours later, an extravasation of cells from the vasculature is noted on the coverslip. Which of the following complement components is the direct cause of the enhanced vascular permeability and chemoattraction in the abraded skin area in this experiment?

C1q
C4b
C5a (Correct answer)
C7
C9

Correct answer: C5a

C5a is a potent anaphylatoxin and chemoattractant produced during complement activation. It directly causes increased vascular permeability by inducing mast cell degranulation and histamine release, which leads to fluid extravasation. Furthermore, C5a is a powerful chemotactic agent, actively recruiting neutrophils and other phagocytes to the site of inflammation. These combined actions directly account for the enhanced vascular permeability and extravasation of cells observed in the experiment.

Question 12: A 35-year-old man comes to the physician for a follow-up examination. He has had persistent left upper quadrant abdominal pain for 3 weeks despite therapy with omeprazole. Upper esophagogastroduodenoscopy shows an active duodenal ulcer. Tests of gastric and duodenal biopsy specimens for urease are positive. Which of the following is the most likely causal organism?

Campylobacter jejuni
Clostridium difficile
Helicobacter pylori (Correct answer)
Proteus vulgaris
Salmonella typhi

Correct answer: Helicobacter pylori

The patient's persistent duodenal ulcer despite omeprazole therapy, coupled with a positive urease test on gastric and duodenal biopsy specimens, is highly indicative of Helicobacter pylori infection. H. pylori is a common cause of peptic ulcers and produces urease, an enzyme that converts urea into ammonia and carbon dioxide, creating a more alkaline environment that allows the bacterium to survive in the acidic stomach. The urease test is a specific diagnostic tool for its presence.

Question 13: A 2-year-old boy is diagnosed with staphylococcal scalded skin syndrome. In vitro studies show the causal organism to be resistant to penicillin. Which of the following mechanisms of action is most likely involved in this resistance?

Active efflux of the antibiotic from the bacteria
Decreased uptake of the antibiotic into the bacteria
Mutation of the 30S ribosomal subunit
Mutation of the 50S ribosomal subunit
Production of β-lactamase by the bacteria (Correct answer)

Correct answer: Production of β-lactamase by the bacteria

Staphylococcus aureus is a common cause of staphylococcal scalded skin syndrome. The most prevalent mechanism of penicillin resistance in S. aureus is the production of β-lactamase (penicillinase). This enzyme hydrolyzes the β-lactam ring of penicillin, rendering the antibiotic inactive and preventing it from interfering with bacterial cell wall synthesis. This resistance mechanism necessitates the use of penicillinase-resistant penicillins or other classes of antibiotics.

Question 14: A 3-year-old girl is brought to the emergency department by her father because of a persistent cough for 2 weeks. She sometimes vomits following a paroxysm of coughing. While she is not coughing, she appears well. She has not yet received any routine childhood vaccinations. Her temperature is 37°C (98.6°F). Physical examination shows petechiae over the sclera and face. Which of the following is the most likely diagnosis?

Diphtheria
Diphtheria
Plague
Tularemia
Pertussis (Correct answer)

Correct answer: Pertussis

The clinical presentation of a persistent cough for two weeks, often followed by vomiting after paroxysms of coughing, in an unvaccinated 3-year-old child is highly characteristic of pertussis, also known as whooping cough. The petechiae over the sclera and face are a result of increased intrathoracic pressure during the severe coughing fits. The child appearing well between coughing episodes is also typical of the paroxysmal stage of pertussis.

Question 15: An investigator injects an experimental animal with a newly discovered bacterial strain to evaluate T-lymphocyte activation. It is found that bacterial engulfment by macrophages results in the presentation of bacterial-derived peptide ligands to CD4+ T lymphocytes. Which of the following cell-surface molecules on the macrophage is most directly involved in the presentation of the processed peptides?

CD28
Class II MHC (Correct answer)
Fcε receptor
Interleukin-2 (IL-2) receptor
Membrane immunoglobulin

Correct answer: Class II MHC

Macrophages are professional antigen-presenting cells (APCs) that engulf extracellular pathogens like bacteria. After processing these bacterial antigens, they present peptide fragments on their cell surface using Class II Major Histocompatibility Complex (MHC) molecules. These MHC Class II-peptide complexes are then recognized by CD4+ T lymphocytes (T helper cells), which is crucial for initiating an adaptive immune response against the pathogen.

Question 16: An investigator conducts an experiment on Clostridium perfringens and then sterilizes the culture dishes by autoclaving. This method of sterilization is most appropriate because it ensures that which of the following bacterial structures are inactivated?

Adhesion factors
Endotoxins
Fimbriae
Pili
Spores (Correct answer)

Correct answer: Spores

Clostridium perfringens is a spore-forming bacterium. Bacterial spores are highly resistant to various environmental stresses, including heat, radiation, and many chemical disinfectants, making them difficult to inactivate. Autoclaving, which uses high-pressure saturated steam at elevated temperatures (typically 121°C), is the most effective method for sterilization because it ensures the destruction and inactivation of even the most resistant bacterial spores, including those of Clostridium species.

Question 17: A 52-year-old woman living in Maryland comes to the physician because of a 1-week history of low-grade fever, fatigue, and a red rash over the skin behind her left knee. She is an avid hiker. Her temperature is 38°C (100.4°F). Physical examination shows an 8-cm, warm, nontender, erythematous lesion with partial central clearing over the skin of the left popliteal area. Which of the following is the most likely causal organism?

Borrelia burgdorferi (Correct answer)
Ehrlichia chaffeensis
Francisella tularensis
Rickettsia rickettsii
Streptobacillus moniliformis

Correct answer: Borrelia burgdorferi

The patient's history of hiking in Maryland, an area endemic for Lyme disease, combined with the characteristic 'bull's-eye' rash (erythema migrans) that is warm, nontender, and has partial central clearing, strongly indicates Lyme disease. Lyme disease is caused by the spirochete Borrelia burgdorferi, transmitted by Ixodes ticks. The associated low-grade fever and fatigue are also consistent early symptoms of this infection.

Question 18: An 8-month-old girl is brought to the emergency department because of a 1-day history of rapid breathing. Her temperature is 38.4°C (101.1°F), pulse is 160/min, respirations are 60/min, and blood pressure is 100/68 mm Hg. Bilateral expiratory wheezing and crackles are heard on auscultation. A chest x-ray shows areas of atelectasis and hyperinflation. Which of the following is the most likely causal virus?

Measles virus
Respiratory syncytial virus (Correct answer)
Rhinovirus
Varicella-zoster virus

Correct answer: Respiratory syncytial virus

The clinical picture of an 8-month-old infant with rapid breathing, fever, bilateral expiratory wheezing and crackles, and chest x-ray findings of atelectasis and hyperinflation is highly characteristic of bronchiolitis. Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis in infants and young children. RSV infection leads to inflammation and obstruction of the small airways, resulting in the observed respiratory distress and lung findings.

Question 19: A 45-year-old man comes to the physician because of fever and night sweats for 8 days. He has Crohn disease treated with infliximab. His temperature is 39°C (102.2°F). Physical examination shows diffuse cervical lymphadenopathy. A biopsy specimen of the nodes shows caseating granulomas. No organisms are identified on Gram stain. Which of the following is the most likely causal organism?

Aspergillus niger
Brucella abortus
Francisella tularensis
Mycobacterium tuberculosis (Correct answer)
Treponema pallidum

Correct answer: Mycobacterium tuberculosis

The patient's symptoms of fever, night sweats, and lymphadenopathy are consistent with tuberculosis. Crucially, his treatment with infliximab, a TNF-α inhibitor, significantly increases the risk of reactivation of latent tuberculosis. The biopsy finding of caseating granulomas is pathognomonic for tuberculosis, even if organisms are not identified on Gram stain (Mycobacterium tuberculosis is acid-fast, not Gram-stain positive). Therefore, Mycobacterium tuberculosis is the most likely causal organism.

Microbiology Test 1

A 45-year-old woman comes to the physician because of progressive facial swelling and pain during the past week.
She has a 10-year history of poorly controlled type 1 diabetes mellitus.
Physical examination shows ecchymoses over the left orbital and periorbital regions with proptosis.
There is a necrotic lesion with a black eschar in the left naris.
Findings on microscopic examination of material from the lesion include broad, irregularly shaped, nonseptate hyphae with branches at right angles.
Which of the following is the most likely cause of the nasal lesion?