Mechanism of action of antimicrobial agents and targets for chemotherapy MCQs With Answer
This quiz collection focuses on the molecular mechanisms of antimicrobial agents and the principal targets exploited in chemotherapy. Designed specifically for M.Pharm students, the 20 MCQs cover bacterial, mycobacterial and fungal targets, including inhibition of cell wall synthesis, protein synthesis, nucleic acid processes, folate metabolism and membrane-active drugs. Questions emphasize biochemical interactions (enzyme inhibition, ribosomal binding sites), prodrug activation and clinically relevant resistance mechanisms such as target modification, enzymatic inactivation and efflux. Items also integrate pharmacodynamic concepts (bactericidal vs bacteriostatic, concentration- vs time-dependent killing) to strengthen mechanistic understanding and support rational antimicrobial selection and therapeutic decision-making.
Q1. Which statement best describes the primary mechanism of action of beta-lactam antibiotics?
- Inhibition of penicillin-binding proteins (transpeptidases) that catalyze peptidoglycan cross-linking
- Binding to D-Ala-D-Ala termini of peptidoglycan precursors, preventing incorporation into the cell wall
- Disruption of cytoplasmic membrane integrity by acting as cationic detergents
- Inhibition of 30S ribosomal subunit preventing aminoacyl-tRNA binding
Correct Answer: Inhibition of penicillin-binding proteins (transpeptidases) that catalyze peptidoglycan cross-linking
Q2. Vancomycin exerts its antibacterial effect primarily by which mechanism?
- Binds to D-Ala-D-Ala termini of peptidoglycan precursors, preventing incorporation into cell wall
- Inhibits DNA gyrase, blocking DNA replication
- Intercalates into bacterial membranes causing rapid depolarization
- Inhibits dihydrofolate reductase, blocking folate synthesis
Correct Answer: Binds to D-Ala-D-Ala termini of peptidoglycan precursors, preventing incorporation into cell wall
Q3. Which characteristic and mechanism are typical of aminoglycoside antibiotics?
- Irreversible binding to the 30S ribosomal subunit causing misreading of mRNA and inhibition of initiation; uptake is oxygen-dependent
- Reversible binding to the 30S ribosomal subunit blocking aminoacyl-tRNA entry
- Inhibition of 50S peptidyl transferase activity, preventing peptide bond formation
- Inhibition of topoisomerase IV, preventing chromosome segregation
Correct Answer: Irreversible binding to the 30S ribosomal subunit causing misreading of mRNA and inhibition of initiation; uptake is oxygen-dependent
Q4. Tetracyclines inhibit bacterial protein synthesis primarily by which action?
- Reversible binding to the 30S ribosomal subunit inhibiting aminoacyl-tRNA attachment to the A site
- Blocking translocation at the 50S subunit
- Preventing formation of the 70S initiation complex
- Irreversible inhibition of peptidyl transferase center
Correct Answer: Reversible binding to the 30S ribosomal subunit inhibiting aminoacyl-tRNA attachment to the A site
Q5. Macrolide antibiotics (e.g., erythromycin) primarily act by which mechanism?
- Binding the 50S ribosomal subunit and blocking translocation of the nascent peptide
- Inhibiting DNA gyrase and topoisomerase IV
- Blocking aminoacyl-tRNA entry at the 30S A site
- Disrupting membrane integrity by binding to LPS
Correct Answer: Binding the 50S ribosomal subunit and blocking translocation of the nascent peptide
Q6. Which antibiotic directly inhibits the peptidyl transferase activity of the 50S ribosomal subunit?
- Chloramphenicol
- Gentamicin
- Tetracycline
- Ciprofloxacin
Correct Answer: Chloramphenicol
Q7. Linezolid’s mechanism of action is best described as:
- Preventing formation of the 70S initiation complex by binding 23S rRNA of the 50S subunit
- Inhibition of DNA-dependent RNA polymerase beta subunit
- Blocking synthesis of ergosterol in fungi
- Disrupting Gram-negative outer membrane by binding LPS
Correct Answer: Preventing formation of the 70S initiation complex by binding 23S rRNA of the 50S subunit
Q8. Fluoroquinolones kill bacteria by targeting which cellular enzymes?
- Inhibition of DNA gyrase (topoisomerase II) and topoisomerase IV, preventing DNA replication
- Inhibition of dihydropteroate synthase in folate synthesis
- Binding to the 30S ribosomal subunit and causing misreading
- Inhibition of peptidoglycan transglycosylases
Correct Answer: Inhibition of DNA gyrase (topoisomerase II) and topoisomerase IV, preventing DNA replication
Q9. Rifampicin (rifampin) exerts its antibacterial effect by:
- Inhibition of DNA-dependent RNA polymerase by binding its β subunit, blocking RNA synthesis
- Inhibition of peptidoglycan cross-linking by binding PBPs
- Disrupting cytoplasmic membranes via cationic interactions
- Blocking aminoacyl-tRNA binding at the ribosomal A site
Correct Answer: Inhibition of DNA-dependent RNA polymerase by binding its β subunit, blocking RNA synthesis
Q10. Sulfonamides and trimethoprim act synergistically because they:
- Sequentially inhibit folate synthesis: sulfonamides inhibit dihydropteroate synthase and trimethoprim inhibits dihydrofolate reductase
- Both inhibit the 30S ribosomal subunit at different binding sites
- Both disrupt bacterial membranes via different cationic interactions
- Inhibit peptidoglycan polymerization and cross-linking respectively
Correct Answer: Sequentially inhibit folate synthesis: sulfonamides inhibit dihydropteroate synthase and trimethoprim inhibits dihydrofolate reductase
Q11. Metronidazole’s selective toxicity for anaerobes is due to:
- Reduction of its nitro group under anaerobic conditions to reactive radicals that damage DNA
- Direct inhibition of bacterial ribosomal peptidyl transferase
- Inhibition of mycolic acid synthesis in anaerobes
- Blocking folate synthesis via dihydropteroate synthase inhibition
Correct Answer: Reduction of its nitro group under anaerobic conditions to reactive radicals that damage DNA
Q12. Polymyxins (e.g., colistin) kill Gram-negative bacteria by:
- Cationic detergent-like disruption of outer and cytoplasmic membranes by binding LPS and phospholipids
- Inhibition of peptidoglycan cross-linking via PBP binding
- Inhibition of DNA gyrase and topo IV
- Blocking the 30S ribosomal A site
Correct Answer: Cationic detergent-like disruption of outer and cytoplasmic membranes by binding LPS and phospholipids
Q13. Daptomycin’s bactericidal action is best described as:
- Calcium-dependent insertion into Gram-positive cytoplasmic membrane causing rapid depolarization and cell death
- Binding to D-Ala-D-Ala and preventing peptidoglycan incorporation
- Inhibition of 50S translocation
- Inhibition of folate synthesis
Correct Answer: Calcium-dependent insertion into Gram-positive cytoplasmic membrane causing rapid depolarization and cell death
Q14. Fosfomycin inhibits an early step in peptidoglycan synthesis by targeting which enzyme?
- MurA (enolpyruvyl transferase)
- Transpeptidase (PBP)
- DNA gyrase
- Dihydrofolate reductase
Correct Answer: MurA (enolpyruvyl transferase)
Q15. Bacitracin interferes with cell wall synthesis by:
- Inhibiting bactoprenol recycling, preventing transport of peptidoglycan subunits across the membrane
- Binding to D-Ala-D-Ala residues of cell wall precursors
- Inhibiting 50S peptidyl transferase activity
- Blocking DNA-directed RNA polymerase
Correct Answer: Inhibiting bactoprenol recycling, preventing transport of peptidoglycan subunits across the membrane
Q16. Isoniazid’s antimycobacterial activity requires activation by which bacterial enzyme, and what is its primary target?
- Activated by KatG and inhibits mycolic acid synthesis (InhA target)
- Activated by beta-lactamase and binds PBPs
- Activated by bacterial sulfatase and inhibits DNA gyrase
- Activated by dihydropteroate synthase and inhibits folate synthesis
Correct Answer: Activated by KatG and inhibits mycolic acid synthesis (InhA target)
Q17. Ethambutol inhibits mycobacterial growth by interfering with:
- Arabinogalactan synthesis via inhibition of arabinosyl transferase
- Mycolic acid activation by KatG
- Protein synthesis by binding the 30S subunit
- Folate metabolism by inhibiting dihydrofolate reductase
Correct Answer: Arabinogalactan synthesis via inhibition of arabinosyl transferase
Q18. Which of the following is NOT a common bacterial mechanism of antibiotic resistance?
- Enzymatic inactivation of the antibiotic (e.g., beta-lactamases)
- Mutation or modification of the drug target (e.g., altered PBPs)
- Upregulation of efflux pumps to reduce intracellular drug concentration
- Increased activation of prodrugs into more toxic metabolites
Correct Answer: Increased activation of prodrugs into more toxic metabolites
Q19. Azole antifungals (e.g., fluconazole) primarily target which enzymatic step in fungi?
- Inhibition of fungal 14α-demethylase (lanosterol demethylase), blocking ergosterol synthesis
- Inhibition of β-(1,3)-glucan synthase
- Direct disruption of fungal cell membrane by cationic detergent action
- Inhibition of fungal DNA-dependent RNA polymerase
Correct Answer: Inhibition of fungal 14α-demethylase (lanosterol demethylase), blocking ergosterol synthesis
Q20. Which antibiotic listed below directly targets the peptidyl transferase center of the bacterial ribosome?
- Chloramphenicol
- Gentamicin
- Tetracycline
- Azithromycin
Correct Answer: Chloramphenicol
