10 USMLE Pharmacology Mechanism Questions with Answers and Explanations (Set 1)

These 10 USMLE pharmacology practice questions focus on how drugs work: first-pass metabolism, receptor antagonists, acid-suppressing drugs, SSRIs, antibiotics, cholinergic drugs, antiretrovirals, beta-blockers and a classic drug interaction. Each question includes the correct answer and an explanation.

Written by Dr. Ray Makar, MD, author of the Repeated USMLE Questions eBook series.

Prefer video? Watch daily USMLE case questions on MedFocus by Dr. Ray on YouTube.

How to use this page: read the question, pick your answer, then tap Show Answer and Explanation.

Questions

Question 1. A 24-year-old medical student visits her physician for a refill on her treatment of depression. She is prescribed imipramine, a tricyclic antidepressant. During the discussion of the treatment, her physician mentions that this drug mainly undergoes first-pass hepatic metabolism. The prescribed drug is most likely administered by which route?

A- Intramuscular
B- Intravenous
C- Oral
D- Subcutaneous
E- Sublingual

First-pass hepatic metabolism refers to the process where a drug is significantly metabolized in the liver before it reaches the systemic circulation. This process is most commonly associated with drugs that are administered orally. When a drug is taken by mouth, it is absorbed through the gastrointestinal tract and then passes through the liver via the portal vein. During this passage, the drug can be metabolized by liver enzymes, which can reduce the amount of active drug that enters the systemic circulation.
Other routes like intramuscular, intravenous, subcutaneous, or sublingual administration generally bypass or minimize first-pass metabolism, leading to more direct absorption into the systemic circulation.
The correct answer is C
Read this Pharmacology MCQs Book

 

Question 2. A scientific researcher is classifying some drugs according to their actions on different receptors. One of these drugs binds to its receptor without producing any action. This drug should be classified as what?

A- Agonist
B- Antagonist
C- Partial agonist
D- Inverse agonist

When a drug binds to its receptor, it competes with other substances which normally affect this receptor causing them to be ineffective. This class of drugs is considered antagonists because they block the action of natural and synthetic agonists. Agonists and partial agonists enhance the action of their receptors either totally or partially. Inverse agonists also activate the receptors but they produce an effect opposite to the action of agonists.
The correct answer is B

 

Question 3. A 45 year-old man presents with progressive burning epigastric pain. He smokes and drinks alcohol socially. Physical examination, laboratory tests and imaging studies confirm the diagnosis of peptic ulcer. He is prescribed omeprazole. What is the effect of this medication that improves this condition?

A- Anti-cholinergic effect
B- Inhibition of H+/K+ ATPase pump
C- Blocking of H2 receptor
D- Neutralization of gastric acid
E- Forming a layer to protect gastric mucosa

Administration of omeprazole leads to irreversible inhibition of the H+/K+ ATPase pump in the parietal cell membrane. Other choices are mechanisms of action of other drugs also used in peptic ulcer medical treatment.
The correct answer is B

 

Question 4. A 33-year-old woman presents to the clinic for her regular yearly physical examination. She has a long history of depression and takes fluoxetine regularly for two years. Which amino acid is the precursor of the neurotransmitter affected by this drug?

A- Glycine
B- Glutamate
C- Methionine
D- Phenylalanine
E- Tryptophan

Fluoxetine is a selective serotonin reuptake inhibitor (SSRI).
Tryptophan is converted to serotonin by tryptophan hydroxylase.
The correct answer is E

 

Question 5. A 54 year-old man with a history of chronic obstructive pulmonary disease (COPD) presents by his son to the emergency room semi-unconscious with shortness of breath. He has nausea, diarrhea and restlessness for the last four hours. His arterial oxygen saturation is 88% with room air. His ECG shows normal sinus rhythm with non-specific changes in lateral leads. The patient uses albuterol inhalation, ipratropium bromide, theophylline and Lisinopril. Three days ago, he had an exacerbation of his COPD and was prescribed ciprofloxacin and supplemental oxygen at night and during ambulation. What is the most likely cause of this patient presentation to the ER?

A- Interaction of ciprofloxacin to albuterol
B- Interaction of ciprofloxacin to ipratropium bromide
C- Interaction of ciprofloxacin to theophylline
D- Interaction of ciprofloxacin to Lisinopril
E- Interaction of ciprofloxacin to oxygen

Ciprofloxacin administration leads to increase in the level or effect of oral theophylline by altering the drug metabolism. Combination may lead to theophylline toxicity and life-threatening interaction. Other drugs mentioned above do not have any undesirable interaction with ciprofloxacin.
The correct answer is C

 

Question 6. A 22-year-old woman presents to the clinic for a recurrent urinary tract infection. She is prescribed trimethoprim/sulfamethoxazole. What is the mechanism of action of this antimicrobial drug?

A- Inhibition of bacterial wall synthesis
B- Inhibition of folic acid synthesis
C- Inhibition of bacterial protein synthesis
D- Inhibition of mycolic acid synthesis
E- Inactivation of bacterial toxins

Sulfonamides act as antimicrobial agents through inhibition of folic acid synthesis, which leads to inhibition of bacterial nucleic acid synthesis. Penicillin and cephalosporin are involved in inhibition of bacterial cell wall synthesis. Inhibition of bacterial protein synthesis is the mechanism of action of aminoglycosides while inhibition of mycolic acid synthesis is the main action of isoniazid. Sulfonamides have no role in inactivation of bacterial toxins.
The correct answer is B

 

Question 7. A 77-year-old man presents to the emergency room with a complaint of retention of urine. Carbachol is administered to stimulate bladder emptying. Which of the following is a prominent side effect of carbachol?

A- Tachycardia
B- Excessive salivation
C- Dry skin
D- Dilation of the pupils
E- Delirium

Carbachol is a cholinomimetic drug which activates acetylcholine receptors so, it is considered cholinergic drug. Cholinergic drugs in general enhance excessive salivation. Stimulation of cholinergic receptors lead to constriction of the pupils, a mechanism helps in glaucoma and as a preoperative drug before cataract surgery. It is also used as stimulant for bladder emptying in cases of urinary retention. It also results in excessive sweating and slow heart rate.
The correct answer is B

 

Question 8. A 23-year-old man with a long history of duodenal ulcer has been treated with cimetidine (Tagamet) for 6 months. What is the mechanism of action of this drug?

A- Adds a protective membrane to gastric mucosa
B- Blocks H2 receptors
C- Stimulates H2 receptors
D- Inhibits H+,K+-ATPase

Cimetidine blocks H2 receptor so, it blocks the secretion of hydrochloric acid. It is H2 receptor antagonist. Proton pump inhibitors such as lansoprazole inhibit H+, K+ ATPase which inhibits acid secretion and has a long duration of action. Sucralfate works by adding a protective membrane to gastric mucosa. H2 receptor stimulators have a very adverse effect on gastric mucosa and they are contraindicated in this condition.
The correct answer is B

 

Question 9. A 24-year-old man with a history of AIDS is currently managed by antiretroviral therapy (HAART) containing a protease-inhibitor, a nucleoside reverse transcriptase inhibitor and a non-nucleoside reverse transcriptase inhibitor. Which one of these drugs is a nucleoside reverse transcriptase inhibitor?

A- Etravirine
B- Efavirenz
C- Indinavir
D- Saquinavir
E- Zidovudine

Zidovudine is the only drug on this list that acts as nucleoside reverse transcriptase inhibitor. Etravirine and efavirenz are non-nucleoside reverse transcriptase inhibitors while indinavir and saquinavir are protease inhibitors.
The correct answer is E

 

Question 10. A 61-year-old man with a history of recurrent angina and hypertension presents to the clinic for routine examination. His vital signs are within normal limits and his cardiac examination is satisfactory. Reviewing his medication, his physician noticed that he had been prescribed labetalol for 2 years. He discussed with his patient replacing it with atenolol. What is the difference between the mechanism of action of atenolol and labetalol?

A- Atenolol is a beta1 blocker while labetalol is a beta 2 blocker
B- Atenolol is a non-selective beta-blocker while labetalol is a beta1 blocker
C- Atenolol is a beta1 blocker while labetalol is both alpha and beta-blocker
D- Atenolol is both beta and alpha-blocker while labetalol is a beta-blocker only
E- Atenolol is a beta 2 blocker while labetalol is a beta1 blocker

Atenolol is a beta1 blocker while labetalol is an alpha and beta-blocker. Beta-blockers, uses, mechanisms of action, side effects and contraindications are very important topics that frequently asked in USMLE.
The correct answer is C

 

High-yield points from this set

  • Drugs given by mouth undergo first-pass hepatic metabolism; IV, IM, SC and sublingual routes bypass it.
  • An antagonist binds the receptor but produces no effect of its own.
  • PPIs such as omeprazole irreversibly inhibit the parietal-cell H+/K+ ATPase.
  • Serotonin is made from tryptophan.
  • Ciprofloxacin inhibits CYP1A2 and raises theophylline levels (toxicity: nausea, seizures, arrhythmias).
  • Sulfonamides and trimethoprim block sequential steps of bacterial folate synthesis.
  • Cholinergic agonists such as carbachol cause salivation, sweating, miosis and bradycardia.
  • Cimetidine is an H2-receptor antagonist (and a CYP450 inhibitor).
  • Zidovudine is a nucleoside reverse transcriptase inhibitor; efavirenz and etravirine are non-nucleoside.
  • Atenolol blocks beta-1 only; labetalol blocks alpha-1, beta-1 and beta-2.

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