USMLE Step 1 Quiz: Antimicrobial Resistance

What this quiz covers

This quiz focuses on Antimicrobial Resistance, 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.

Question 1

1. Production of beta-lactamase that hydrolyzes the antibiotic.
2. Alteration of the drug's target site. (correct answer)
3. Increased efflux of the antibiotic out of the cell.
4. Decreased permeability of the cell wall to the antibiotic.

Explanation: The patient has methicillin-resistant Staphylococcus aureus (MRSA), indicated by nafcillin resistance and the presence of the mecA gene. This gene encodes for a modified penicillin-binding protein (PBP2a), which has a low affinity for beta-lactam antibiotics. This alteration of the target site prevents the antibiotic from effectively inhibiting cell wall synthesis.

Question 2

1. Methylation of the 23S ribosomal RNA.
2. Enzymatic inactivation of vancomycin by phosphorylation.
3. Modification of the peptidoglycan precursor target. (correct answer)
4. Expression of an efflux pump that removes vancomycin from the cell.

Explanation: Vancomycin-resistant enterococci (VRE) with the vanA gene cluster exhibit resistance by altering the drug's target. The terminal D-alanyl-D-alanine (D-Ala-D-Ala) of the peptidoglycan precursor is modified to D-alanyl-D-lactate (D-Ala-D-Lac). This change significantly reduces the binding affinity of vancomycin, rendering it ineffective.

Question 3

1. Mutation of porin channels preventing drug entry.
2. Production of an extended-spectrum beta-lactamase. (correct answer)
3. Alteration of penicillin-binding proteins.
4. Modification of the 30S ribosomal subunit.

Explanation: The susceptibility pattern (resistance to third-generation cephalosporins like ceftriaxone but susceptibility to carbapenems) and a positive double-disk synergy test are characteristic of an extended-spectrum beta-lactamase (ESBL)-producing organism. ESBLs are enzymes that hydrolyze and inactivate penicillins and extended-spectrum cephalosporins.

Question 4

1. Outer membrane porin structure.
2. DNA gyrase. (correct answer)
3. 30S ribosomal subunit.
4. Dihydropteroate synthase.

Explanation: Fluoroquinolones, such as ciprofloxacin, exert their effect by inhibiting bacterial DNA gyrase (topoisomerase II) and topoisomerase IV. Resistance commonly arises from point mutations in the genes encoding these enzymes, such as gyrA for DNA gyrase. These mutations alter the drug's binding site, reducing its inhibitory effect.

Question 5

1. Formation of a drug-inactivating enzyme. (correct answer)
2. Mutation of the 30S ribosomal subunit.
3. Decreased drug uptake due to porin loss.
4. Modification of the cell membrane lipid A.

Explanation: A common and highly effective mechanism of resistance to aminoglycosides like gentamicin is enzymatic modification of the drug. Bacteria can acquire plasmids encoding enzymes that inactivate aminoglycosides through acetylation, phosphorylation, or adenylation. This modification prevents the drug from binding to its target, the 30S ribosomal subunit.

Question 6

1. Methylation of the 23S rRNA binding site.
2. Active drug efflux. (correct answer)
3. Phosphorylation of the antibiotic.
4. Alteration of penicillin-binding proteins.

Explanation: The mef (macrolide efflux) gene encodes a drug efflux pump that actively transports macrolides out of the bacterial cell, keeping intracellular concentrations below effective levels. This results in the M phenotype of resistance (resistant to macrolides but susceptible to lincosamides like clindamycin), as opposed to the erm gene which causes methylation of 23S rRNA and results in the MLS-B phenotype (resistance to macrolides, lincosamides, and streptogramin B).

Question 7

1. DNA-dependent RNA polymerase. (correct answer)
2. Mycolic acid synthesis pathway.
3. 30S ribosomal subunit.
4. DNA gyrase.

Explanation: Rifampin functions by inhibiting bacterial DNA-dependent RNA polymerase, thereby blocking transcription. The rpoB gene encodes the beta subunit of this enzyme. Mutations in this gene are the primary mechanism of rifampin resistance in M. tuberculosis, as they alter the drug-binding site on the RNA polymerase, preventing the drug's inhibitory action.

Question 8

1. Ribosomal protection by a protein that displaces the drug.
2. Enzymatic inactivation of the drug by hydroxylation.
3. Creation of an energy-dependent efflux pump. (correct answer)
4. Mutation in the 16S rRNA component of the 30S ribosome.

Explanation: The most common mechanism of tetracycline resistance is the acquisition of genes, such as tet(A), that encode for membrane-associated efflux pumps. These pumps use energy (e.g., proton motive force) to actively transport tetracycline molecules out of the bacterial cell, preventing the drug from reaching the high intracellular concentration needed to inhibit protein synthesis at the 30S ribosome.

Question 9

1. Dihydropteroate synthase.
2. DNA gyrase.
3. Dihydrofolate reductase. (correct answer)
4. RNA polymerase.

Explanation: Trimethoprim inhibits bacterial folate synthesis by targeting dihydrofolate reductase (DHFR). A primary mechanism of resistance is the acquisition of a plasmid-borne gene (such as a dfr gene) that encodes a modified DHFR. This altered enzyme is highly resistant to inhibition by trimethoprim but retains its normal enzymatic function, allowing the bacteria to continue synthesizing tetrahydrofolate and survive.

Question 10

1. Decreased drug uptake due to altered porins.
2. Enzymatic inactivation by acetylation. (correct answer)
3. Mutation of the 50S ribosomal subunit.
4. Active efflux of the drug from the bacterial cell.

Explanation: The most common mechanism of plasmid-mediated resistance to chloramphenicol is enzymatic inactivation. The plasmid carries the gene for chloramphenicol acetyltransferase (CAT). This enzyme transfers an acetyl group from acetyl-CoA to the chloramphenicol molecule, rendering it unable to bind to the 50S ribosomal subunit and inhibit protein synthesis.

Question 11

1. Transformation.
2. Generalized transduction.
3. Conjugation. (correct answer)
4. Spontaneous chromosomal mutation.

Explanation: The scenario describes the transfer of a resistance gene on a large, self-transmissible extrachromosomal DNA element (a plasmid) between different bacterial species (Acinetobacter and Klebsiella). This process is conjugation, which involves the transfer of genetic material, typically a plasmid, from a donor to a recipient cell through direct cell-to-cell contact via a sex pilus. It is a major mechanism for the spread of antibiotic resistance among gram-negative bacteria.

Question 12

1. Hemagglutinin.
2. M2 proton channel.
3. Neuraminidase. (correct answer)
4. RNA-dependent RNA polymerase.

Explanation: Oseltamivir is a neuraminidase inhibitor. Neuraminidase is a viral enzyme that cleaves sialic acid residues, allowing newly formed virions to be released from the infected host cell. The H275Y mutation occurs in the neuraminidase enzyme, altering its active site. This change reduces the binding affinity of oseltamivir, allowing the enzyme to function despite the presence of the drug, leading to resistance.

Question 13

1. Alteration of the target enzyme, lanosterol 14-alpha-demethylase.
2. Increased production of drug efflux pumps. (correct answer)
3. Decreased ergosterol content in the cell membrane.
4. Formation of a biofilm that is impermeable to the drug.

Explanation: A major mechanism of azole resistance in Candida albicans is the upregulation of genes encoding ATP-binding cassette (ABC) transporters (e.g., CDR1, CDR2) and major facilitator superfamily (MFS) transporters (e.g., MDR1). These transporters function as multidrug efflux pumps, actively removing azole antifungals from the cell, which prevents them from reaching their target, lanosterol 14-alpha-demethylase.

Question 14

1. Bacteriophage.
2. Transposon. (correct answer)
3. Insertion sequence.
4. Pathogenicity island.

Explanation: The vanA gene cluster, which confers high-level vancomycin resistance, is often located on a transposon (e.g., Tn1546). Transposons are mobile genetic elements ('jumping genes') that can move from one location to another within the genome, such as from a plasmid to a chromosome or between plasmids. This mobility greatly facilitates the dissemination of resistance genes.

Question 15

1. It prevents the binding of the drug to its ultimate target.
2. It leads to overexpression of a drug efflux pump.
3. It impairs the enzymatic activation of the prodrug. (correct answer)
4. It alters the structure of the mycobacterial cell wall.

Explanation: Isoniazid is a prodrug that must be activated within the mycobacterium to become effective. The bacterial enzyme catalase-peroxidase, encoded by the katG gene, is responsible for this activation. Mutations that inactivate or reduce the function of KatG prevent the conversion of isoniazid to its active form. As a result, the drug cannot exert its therapeutic effect, which is the inhibition of mycolic acid synthesis.

Question 16

1. Production of a carbapenemase. (correct answer)
2. Decreased expression of the catalase-peroxidase enzyme.
3. Mutation in the gene encoding RNA polymerase.
4. Thickening of the peptidoglycan cell wall.

Explanation: Resistance to carbapenems (e.g., imipenem, meropenem) in Klebsiella pneumoniae is most commonly due to the production of carbapenemases, such as Klebsiella pneumoniae carbapenemase (KPC). These enzymes are beta-lactamases that can hydrolyze a broad spectrum of beta-lactam antibiotics, including the carbapenems, rendering them ineffective.

Question 17

1. Modification of the antibiotic target.
2. Enzymatic hydrolysis of the antibiotic.
3. Reduced drug influx. (correct answer)
4. Enhanced drug efflux.

Explanation: Pseudomonas aeruginosa is an opportunistic pathogen known for its intrinsic and acquired resistance. One important mechanism is its relatively impermeable outer membrane. OprD is a specific outer membrane porin channel through which carbapenems preferentially enter the cell. Downregulation or mutational inactivation of the oprD gene leads to decreased permeability and reduced influx of the drug, resulting in resistance.

Question 18

1. Production of an alternative dihydropteroate synthase. (correct answer)
2. Increased production of para-aminobenzoic acid (PABA).
3. Decreased permeability to the drug.
4. Enzymatic degradation of sulfonamide.

Explanation: Sulfonamides are structural analogs of PABA and competitively inhibit dihydropteroate synthase, a key enzyme in the bacterial folate synthesis pathway. The most common mechanism of resistance is the acquisition of a plasmid-encoded gene (e.g., sul1, sul2) that produces an altered dihydropteroate synthase. This modified enzyme has a much lower affinity for sulfonamides but can still efficiently use PABA, thus bypassing the drug's inhibitory effect.

Question 19

1. Alteration of D-Ala-D-Ala to D-Ala-D-Lac.
2. Production of a beta-lactamase specific for vancomycin.
3. Trapping of vancomycin molecules in the outer layers of the cell wall. (correct answer)
4. Efflux of vancomycin via a membrane pump.

Explanation: This patient has vancomycin-intermediate S. aureus (VISA). The mechanism of resistance is distinct from the high-level resistance seen in VRE. In VISA, there are mutations that lead to a disordered, thickened cell wall with an increased number of D-Ala-D-Ala residues. Vancomycin molecules get trapped in this thick outer layer, binding to these false targets and are thus prevented from reaching their site of action at the cell membrane.

Question 20

1. Modification of porin channels.
2. Production of beta-lactamase. (correct answer)
3. Alteration of DNA gyrase.
4. Modification of penicillin-binding proteins.

Explanation: Neisseria gonorrhoeae has developed resistance to penicillins through multiple mechanisms. A historically important and common plasmid-mediated mechanism is the production of a beta-lactamase enzyme (penicillinase) that cleaves the beta-lactam ring of penicillin, inactivating the drug. Chromosomal mutations leading to altered PBPs or porin channels also contribute to resistance but beta-lactamase production is the classic plasmid-mediated mechanism.