USMLE Pharmacology 1 — Questions and Answers
Question 1: 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 2: Which beta-blocker is cardioselective and preferred in a patient with COPD requiring beta-blockade?
- Metoprolol (Correct answer)
- Propranolol
- Carvedilol
- Labetalol
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 3: A patient with type 2 diabetes develops lactic acidosis. Which medication is most likely responsible?
- Metformin (Correct answer)
- Glipizide
- Sitagliptin
- Pioglitazone
Correct answer: Metformin
Metformin can cause lactic acidosis, particularly in patients with renal insufficiency, hepatic disease, or conditions causing tissue hypoperfusion.
Question 4: Which antiepileptic drug works by blocking voltage-gated sodium channels and is also used for neuropathic pain and bipolar disorder?
- Carbamazepine (Correct answer)
- Valproate
- Ethosuximide
- Phenobarbital
Correct answer: Carbamazepine
Carbamazepine blocks voltage-gated sodium channels and is used for partial seizures, trigeminal neuralgia, and bipolar disorder.
Question 5: A patient develops gynecomastia while taking spironolactone. What is the mechanism of this side effect?
- Antagonism of androgen receptors (Correct answer)
- Inhibition of 5-alpha reductase
- Inhibition of CYP17A1
- Stimulation of estrogen receptors
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 6: Which ACE inhibitor side effect is class-specific and due to bradykinin accumulation?
- Dry cough (Correct answer)
- Hyperkalemia
- First-dose hypotension
- Angioedema
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.
USMLE Pharmacology 1
A patient on warfarin develops supratherapeutic INR after starting a new antibiotic.
Which mechanism best explains this interaction?