USMLE Step 1 Quiz: Antimicrobial Pharmacology
20 questions · exam conditions
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Antimicrobial PharmacologyQuestion 1 of 20

Why does gentamicin fail in an anaerobic abscess?

No oxygen for drug uptake
Beta-lactamase inactivates it
Efflux pump removes the drug
Ribosomal target is mutated
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USMLE Step 1 Quiz

USMLE Step 1 Quiz: Antimicrobial Pharmacology

Practice Antimicrobial Pharmacology in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Antimicrobial Pharmacology, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

Why does gentamicin fail in an anaerobic abscess?

  1. No oxygen for drug uptake (correct answer)
  2. Beta-lactamase inactivates it
  3. Efflux pump removes the drug
  4. Ribosomal target is mutated
Explanation: Gentamicin needs oxygen-dependent active transport to enter bacteria. In an anaerobic abscess there is no oxygen, so the drug cannot get inside to reach its ribosomal target. The tempting wrong answer is a mutated ribosomal target, but the failure is lack of uptake, not target alteration.

Question 2

Which index best predicts beta-lactam efficacy?

  1. Peak-to-MIC ratio
  2. Time above the MIC (correct answer)
  3. AUC/MIC ratio value
  4. Postantibiotic effect
Explanation: Beta-lactams kill best when drug levels stay above the MIC for a sufficient portion of the dosing interval, so time above the MIC is the pharmacodynamic index that predicts their efficacy. The tempting wrong answer is AUC/MIC, but that index applies mainly to concentration-dependent killers such as aminoglycosides and fluoroquinolones, not beta-lactams.

Question 3

What mechanism best explains vancomycin-intermediate S. aureus (VISA)?

  1. Thick cell wall traps drug (correct answer)
  2. VanA ligase alters target
  3. PBP2a with low affinity
  4. Beta-lactamase alters wall
Explanation: VISA strains build a thickened cell wall with excess dipeptide targets that bind vancomycin before it reaches the active site, effectively trapping the drug. This lowers the amount of drug reaching its target, so susceptibility falls. The tempting wrong answer is VanA ligase altering the target, but that causes high-level VRSA, not VISA. PBP2a and beta-lactamase explain beta-lactam resistance, not vancomycin resistance.

Question 4

Which resistance mechanism is inhibited by clavulanate?

  1. PBP2a target change
  2. AmpC beta-lactamases
  3. VanA ligase alteration
  4. ESBL beta-lactamases (correct answer)
Explanation: Clavulanate blocks beta-lactamase enzymes that would otherwise destroy penicillins, and extended-spectrum beta-lactamases (ESBLs) are among the enzymes it inhibits. The tempting wrong answer is AmpC beta-lactamases, but AmpC enzymes are generally not inhibited by clavulanate, so that mechanism does not fit.

Question 5

What does linezolid added to an SSRI most likely cause?

  1. Severe lactic acidosis
  2. Hypertensive crisis
  3. Serotonin syndrome (correct answer)
  4. Peripheral neuropathy
Explanation: Linezolid inhibits monoamine oxidase, and adding it to an SSRI traps serotonin, so you get serotonin syndrome with fever, clonus, and agitation. The tempting pick is hypertensive crisis, but that requires tyramine-rich foods or sympathomimetics, not an SSRI.

Question 6

A Gram-negative rod is ceftriaxone resistant; clavulanate restores susceptibility. Which mechanism is present?

  1. AmpC beta-lactamase present
  2. ESBL beta-lactamase present (correct answer)
  3. KPC carbapenemase present
  4. Outer membrane porin loss
Explanation: Clavulanate inhibits ESBL enzymes, so ceftriaxone resistance that disappears when clavulanate is added points to ESBL. AmpC is the tempting wrong answer, but clavulanate does not reliably inhibit AmpC beta-lactamases, so it would not restore susceptibility. KPC carbapenemase and porin loss also do not fit this clavulanate effect.

Question 7

A staphylococcal isolate is erythromycin resistant, clindamycin susceptible, D-zone positive. What is the mechanism?

  1. Constitutive erm methylase
  2. MsrA efflux pump present
  3. Inducible erm methylase (correct answer)
  4. Altered ribosomal protein
Explanation: D-zone positive means erythromycin induces the erm methylase, which methylates the ribosomal target and makes clindamycin ineffective even though the isolate initially looks susceptible. Constitutive erm methylase would show resistance to both drugs without induction, and an efflux pump causes erythromycin resistance without affecting clindamycin.

Question 8

Why does adding gentamicin to ampicillin produce synergy in enterococcal endocarditis?

  1. Blocks folate synthesis
  2. Both bind 50S ribosome
  3. Prevents ampicillin efflux
  4. Wall disruption aids uptake (correct answer)
Explanation: Ampicillin weakens the enterococcal cell wall, making it easier for gentamicin to enter and bind its 30S ribosomal target. This overcomes enterococci poor aminoglycoside uptake and creates bactericidal synergy. The tempting wrong choice is that both drugs bind the 50S ribosome; gentamicin binds 30S and ampicillin blocks cell wall synthesis, not protein synthesis.

Question 9

A patient on rifampin has a falling INR while taking warfarin. What explains this?

  1. Inhibits CYP3A4 metabolism
  2. Induces renal excretion
  3. Induces CYP3A4 metabolism (correct answer)
  4. Displaces protein binding
Explanation: Rifampin is a potent enzyme inducer, so it speeds up warfarin metabolism and clearance, lowering the INR. It induces CYP2C9, which clears the more potent S-warfarin enantiomer, along with CYP3A4 and CYP1A2, which handle R-warfarin. The tempting wrong answer is protein-binding displacement, but that only transiently raises free warfarin and would raise, not lower, the INR. Chronic enzyme induction is what explains the falling INR.

Question 10

An isolate has MIC 0.5 µg/mL and MBC 32 µg/mL. What best describes the agent?

  1. Bacteriostatic at 0.5 µg/mL (correct answer)
  2. Bactericidal at 0.5 µg/mL
  3. Shows time-dependent killing
  4. Post-antibiotic effect seen
Explanation: The MBC is 64-fold higher than the MIC (32 / 0.5 = 64), so the concentration that stops growth does not kill the organism; the drug is bacteriostatic at the MIC. Bactericidal at 0.5 is tempting, but bactericidal agents have an MBC close to the MIC, usually within 4-fold, not 64-fold apart.

Question 11

A 65-year-old man with a prosthetic heart valve is admitted with fever and malaise. Blood cultures grow methicillin-resistant Staphylococcus aureus (MRSA). He is started on intravenous vancomycin. During the first infusion, he develops diffuse erythema over his face, neck, and upper torso, along with pruritus and hypotension.

The therapeutic effect of the medication responsible for this reaction is achieved by which of the following mechanisms?

  1. Binding directly to D-alanyl-D-alanine moieties of peptidoglycan precursors (correct answer)
  2. Inhibiting translocation of the peptide chain on the 50S ribosomal subunit
  3. Forming pores in the bacterial cell membrane via lipophilic insertion
  4. Inhibiting the bacterial enzyme DNA-dependent RNA polymerase
Explanation: Vancomycin is a glycopeptide antibiotic that inhibits cell wall synthesis in gram-positive bacteria by binding to the D-Ala-D-Ala terminus of peptidoglycan precursors. This sterically hinders transglycosylase and transpeptidase, preventing cell wall elongation and cross-linking. The patient's symptoms are characteristic of red man syndrome, a rate-dependent infusion reaction caused by widespread histamine release, which is a common side effect of vancomycin.

Question 12

A 72-year-old woman in the intensive care unit for urosepsis develops a ventilator-associated pneumonia caused by Pseudomonas aeruginosa. She is treated with intravenous gentamicin. One week into therapy, she complains of ringing in her ears and difficulty hearing. Her serum creatinine has also increased.

The medication responsible for these adverse effects exerts its antimicrobial activity by which of the following mechanisms?

  1. Inhibiting bacterial cell wall synthesis
  2. Interfering with folate synthesis
  3. Binding irreversibly to the 30S ribosomal subunit, causing mRNA misreading (correct answer)
  4. Binding to the 50S ribosomal subunit, inhibiting peptide bond formation
Explanation: Gentamicin is an aminoglycoside. These antibiotics bind irreversibly to the 16S rRNA of the 30S ribosomal subunit, which leads to misreading of the mRNA codon and the production of faulty proteins, as well as premature termination of translation. Ototoxicity and nephrotoxicity are well-known adverse effects of aminoglycosides.

Question 13

A 48-year-old woman with a history of type 1 diabetes mellitus presents with fever, chills, flank pain, and dysuria. Urinalysis is consistent with pyelonephritis, and urine cultures grow a gram-negative rod. She is admitted and started on intravenous ceftriaxone.

The bactericidal effect of ceftriaxone is due to its ability to inhibit which of the following processes?

  1. Protein synthesis
  2. Folate metabolism
  3. Peptidoglycan cross-linking (correct answer)
  4. Nucleic acid synthesis
Explanation: Ceftriaxone is a third-generation cephalosporin, which is a class of beta-lactam antibiotics. Like other beta-lactams, its mechanism of action involves binding to and inactivating penicillin-binding proteins (PBPs), primarily transpeptidases. This inhibition prevents the final cross-linking step of peptidoglycan synthesis, compromising the integrity of the bacterial cell wall and leading to bacteriolysis.

Question 14

A 50-year-old man with advanced HIV/AIDS presents with headache, fever, and neck stiffness. A lumbar puncture reveals an elevated opening pressure and India ink stain shows encapsulated yeasts. He is diagnosed with cryptococcal meningitis and started on intravenous amphotericin B. Two days later, his serum creatinine rises significantly.

The nephrotoxicity of the administered drug is primarily caused by its binding to which of the following molecules in human renal tubular cells?

  1. Cholesterol (correct answer)
  2. Peptidoglycan
  3. Beta-glucan
  4. 50S ribosomal subunit
Explanation: Amphotericin B's primary antifungal mechanism is binding to ergosterol, the main sterol in fungal cell membranes, forming pores that lead to ion leakage and cell death. However, it can also bind to cholesterol in mammalian cell membranes, albeit with lower affinity. This binding in human renal tubular cells disrupts membrane integrity, leading to electrolyte wasting (hypokalemia, hypomagnesemia) and renal vasoconstriction, which together cause its characteristic nephrotoxicity.

Question 15

A 45-year-old man presents with thickened, yellow, and crumbling toenails. A potassium hydroxide preparation of nail clippings reveals fungal hyphae. He is diagnosed with onychomycosis and prescribed a 12-week course of oral terbinafine.

This drug is effective against dermatophytes because it inhibits which of the following fungal enzymes?

  1. 14-alpha-demethylase
  2. Beta-(1,3)-D-glucan synthase
  3. Squalene epoxidase (correct answer)
  4. Thymidylate synthase
Explanation: Terbinafine is an allylamine antifungal that works by inhibiting squalene epoxidase, a key enzyme in the ergosterol synthesis pathway. This inhibition leads to a deficiency of ergosterol and a toxic accumulation of squalene within the fungal cell, which disrupts membrane function and leads to cell death. Terbinafine is highly lipophilic and keratophilic, allowing it to concentrate in nails and skin.

Question 16

A 60-year-old hematopoietic stem cell transplant recipient develops fever and cough unresponsive to broad-spectrum antibacterial agents. A CT scan of the chest reveals nodules with a 'halo sign,' and serum galactomannan is elevated, consistent with invasive aspergillosis. He is started on intravenous caspofungin.

What is the mechanism of action of this antifungal agent?

  1. Binding to ergosterol to form pores in the cell membrane
  2. Inhibition of the synthesis of beta-(1,3)-D-glucan (correct answer)
  3. Inhibition of the fungal enzyme squalene epoxidase
  4. Interference with fungal microtubule function
Explanation: Caspofungin is an echinocandin antifungal. Echinocandins work by noncompetitively inhibiting the enzyme beta-(1,3)-D-glucan synthase. This disrupts the synthesis of beta-glucan, an essential polysaccharide component of the fungal cell wall (but not human cells). The resulting weakened cell wall leads to osmotic instability and fungal cell death.

Question 17

A 70-year-old man hospitalized with aspiration pneumonia develops a subsequent infection with a multi-drug resistant gram-negative organism. He is started on a regimen that includes imipenem combined with cilastatin.

What is the primary purpose of co-administering cilastatin with imipenem?

  1. To inhibit bacterial beta-lactamase enzymes
  2. To decrease the risk of seizures associated with imipenem
  3. To inhibit a human renal enzyme that metabolizes imipenem (correct answer)
  4. To enhance the penetration of imipenem into the CSF
Explanation: Imipenem, a broad-spectrum carbapenem antibiotic, is rapidly inactivated in the renal tubules by a human enzyme called dehydropeptidase I. Cilastatin is an inhibitor of this enzyme. By co-administering cilastatin, the inactivation of imipenem is blocked, which increases its half-life and allows it to achieve effective therapeutic concentrations in the urine and systemic circulation.

Question 18

A 19-year-old college student presents with a sore throat, fever, and tonsillar exudates. A rapid streptococcal antigen test is positive, and he is diagnosed with streptococcal pharyngitis. He is prescribed a 10-day course of amoxicillin. The patient's symptoms resolve within three days.

Which of the following best describes the mechanism of action of the prescribed medication?

  1. Inhibition of transpeptidase-catalyzed cell wall cross-linking (correct answer)
  2. Binding to the 30S ribosomal subunit to block protein synthesis
  3. Inhibition of bacterial DNA gyrase and topoisomerase IV
  4. Creation of free radicals that cause DNA damage
Explanation: Amoxicillin is a beta-lactam antibiotic. Beta-lactams work by irreversibly inhibiting transpeptidase (also known as penicillin-binding protein), an enzyme crucial for the final step of peptidoglycan synthesis. This inhibition weakens the bacterial cell wall, leading to cell lysis and death.

Question 19

A 68-year-old woman with a history of recurrent urinary tract infections is prescribed ciprofloxacin for pyelonephritis. Several days into the treatment, she develops severe pain in her posterior ankle, making it difficult to walk. Physical examination reveals tenderness and swelling over the Achilles tendon.

The drug responsible for this patient's symptoms acts by inhibiting which of the following?

  1. Transpeptidase
  2. DNA gyrase and topoisomerase IV (correct answer)
  3. Dihydrofolate reductase
  4. DNA-dependent RNA polymerase
Explanation: Ciprofloxacin is a fluoroquinolone antibiotic. Fluoroquinolones exert their bactericidal effect by inhibiting bacterial DNA gyrase (topoisomerase II) and topoisomerase IV, enzymes necessary for DNA replication, transcription, and repair. A well-known, though rare, adverse effect of fluoroquinolones is tendonitis and tendon rupture, particularly of the Achilles tendon.

Question 20

A 45-year-old man with HIV and a CD4 count of 80 cells/mm³ is diagnosed with Pneumocystis jirovecii pneumonia (PCP). He is started on high-dose trimethoprim-sulfamethoxazole (TMP-SMX).

This combination therapy is effective due to its ability to sequentially inhibit the synthesis of which of the following essential molecules?

  1. Ergosterol
  2. Peptidoglycan
  3. Mycolic acid
  4. Tetrahydrofolate (correct answer)
Explanation: TMP-SMX works by sequentially inhibiting two key enzymes in the bacterial and protozoal folate synthesis pathway. Sulfamethoxazole (a sulfonamide) competes with para-aminobenzoic acid (PABA) to inhibit dihydropteroate synthase. Trimethoprim inhibits dihydrofolate reductase. This dual blockade prevents the synthesis of tetrahydrofolate, a crucial cofactor for purine and thymidine synthesis, leading to a bactericidal effect.