Three USMLE Step 1 pharmacology questions on a centrally acting antihypertensive, a classic H2-blocker side effect and an important pharmacology definition. Pick your answer first, then open the explanation.
Question 1: Which antihypertensive acts on the central vasomotor center?
Which of the following drugs reduces blood pressure by acting centrally on the vasomotor center?
A. Atenolol B. Clonidine C. Enalapril D. Nifedipine
Correct answer: B. Clonidine
Clonidine is a central alpha-2 adrenergic agonist. It reduces sympathetic outflow from the brainstem vasomotor center, lowering heart rate, cardiac output and peripheral resistance. It is not a first-line drug but is used for resistant hypertension, ADHD and opioid withdrawal symptoms.
Why the other options are wrong:
Atenolol is a cardioselective beta-1 blocker that acts peripherally on the heart.
Enalapril is an ACE inhibitor that blocks formation of angiotensin II.
Nifedipine is a dihydropyridine calcium channel blocker that dilates arterioles.
High-yield: Stopping clonidine suddenly causes rebound hypertension. Methyldopa is another central alpha-2 agonist and is safe in pregnancy.
Question 2: Which drug can cause reversible gynecomastia?
Which of the following drugs may cause reversible gynecomastia?
A. Cimetidine B. Omeprazole C. Misoprostol D. Sucralfate
Correct answer: A. Cimetidine
Cimetidine, an H2 receptor blocker, has anti-androgenic effects (it blocks androgen receptors and increases prolactin), which can cause gynecomastia, impotence and decreased libido. It also inhibits cytochrome P450, causing many drug interactions.
Why the other options are wrong:
Omeprazole is a proton pump inhibitor; long-term use is linked to low magnesium, vitamin B12 deficiency, fractures and C. difficile infection.
Misoprostol is a PGE1 analog; it causes diarrhea and is contraindicated in pregnancy (abortifacient).
Sucralfate coats the ulcer base; its main side effect is constipation.
High-yield: Other drugs causing gynecomastia: spironolactone, ketoconazole, digoxin, estrogens and cimetidine.
Question 3: What is tachyphylaxis?
Which of the following is the most accurate definition of tachyphylaxis?
A. A decrease in the effect of a drug after use for a prolonged time B. A drug interaction between two drugs C. A toxic effect after repeated use of the same drug in a short period of time D. Very rapidly developing tolerance to a drug
Correct answer: D. Very rapidly developing tolerance to a drug
Tachyphylaxis is a rapid decrease in response to a drug after repeated doses given over a short time. A classic example is ephedrine or amphetamine, which release stored norepinephrine; once the stores are depleted, repeated doses produce less effect.
Why the other options are wrong:
A describes ordinary tolerance, which develops gradually over a long time.
B describes a drug interaction, not tachyphylaxis.
C describes cumulative toxicity, not a reduced response.
High-yield: Other examples of tachyphylaxis: nitroglycerin (needs a daily nitrate-free interval) and nasal decongestants such as oxymetazoline (rebound congestion).
Key takeaways
Clonidine → central alpha-2 agonist; rebound hypertension if stopped abruptly.
Cimetidine → gynecomastia (anti-androgen) and CYP450 inhibition.
Tachyphylaxis → rapid tolerance over a short time (e.g. nitroglycerin, ephedrine).
