Antimicrobials: Mechanism of action MCQs With Answer

Antimicrobials: Mechanism of action MCQs With Answer — Mastering antimicrobial mechanisms is essential for B. Pharm students preparing for exams and safe clinical practice. This focused set of MCQs covers antimicrobial pharmacology, antibiotic mechanisms, antifungal and antiviral drug targets, resistance mechanisms, and PK/PD concepts linked to mechanism of action. Questions delve into cell wall synthesis inhibitors, protein synthesis blockade, nucleic acid synthesis disruption, membrane-active agents, folate pathway inhibitors, and mechanism-based adverse effects. Each item reinforces critical concepts like MIC, MBC, time- versus concentration-dependent killing, and common resistance strategies. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which class of antibiotics inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins and blocking transpeptidation?

• Macrolides
• Beta-lactams
• Tetracyclines
• Fluoroquinolones

Correct Answer: Beta-lactams

Q2. Vancomycin primarily exerts its bactericidal effect by:

• Inhibiting DNA gyrase
• Binding to D-Ala-D-Ala termini and preventing peptidoglycan cross-linking
• Blocking the 30S ribosomal subunit
• Disrupting bacterial cell membrane by forming pores

Correct Answer: Binding to D-Ala-D-Ala termini and preventing peptidoglycan cross-linking

Q3. Aminoglycosides cause bacterial cell death by:

• Inhibiting folate synthesis
• Inducing misreading of mRNA at the 30S ribosomal subunit
• Blocking DNA-dependent RNA polymerase
• Inhibiting beta-lactamase enzymes

Correct Answer: Inducing misreading of mRNA at the 30S ribosomal subunit

Q4. Which mechanism best explains how fluoroquinolones kill bacteria?

• Inhibition of peptidoglycan polymerization
• Inhibition of DNA topoisomerases (DNA gyrase and topoisomerase IV)
• Disruption of membrane integrity via binding ergosterol
• Inhibition of dihydropteroate synthase

Correct Answer: Inhibition of DNA topoisomerases (DNA gyrase and topoisomerase IV)

Q5. Sulfonamides primarily act by:

• Inhibiting folate synthesis via competitive inhibition of dihydropteroate synthase
• Blocking peptidyl transferase at the 50S ribosome
• Binding to ergosterol in fungal membranes
• Inhibiting neuraminidase in influenza

Correct Answer: Inhibiting folate synthesis via competitive inhibition of dihydropteroate synthase

Q6. Which antifungal agent binds ergosterol and creates membrane pores leading to cell death?

• Fluconazole
• Amphotericin B
• Terbinafine
• Caspofungin

Correct Answer: Amphotericin B

Q7. Macrolides like erythromycin inhibit bacterial growth by:

• Binding to the 50S ribosomal subunit and blocking translocation
• Inhibiting DNA replication by targeting DNA gyrase
• Disrupting cell membrane integrity by interacting with LPS
• Blocking folate uptake into bacteria

Correct Answer: Binding to the 50S ribosomal subunit and blocking translocation

Q8. Which antiviral drug is a nucleoside analogue that requires activation by viral thymidine kinase and inhibits viral DNA polymerase?

• Oseltamivir
• Acyclovir
• Ribavirin
• Zanamivir

Correct Answer: Acyclovir

Q9. Daptomycin kills Gram-positive bacteria by:

• Inhibiting cell wall synthesis like beta-lactams
• Depolarizing the bacterial membrane leading to rapid potassium efflux
• Blocking the 30S ribosomal subunit
• Inhibiting DNA gyrase

Correct Answer: Depolarizing the bacterial membrane leading to rapid potassium efflux

Q10. Which mechanism is responsible for bacterial resistance by enzymatic inactivation of beta-lactam antibiotics?

• Efflux pump overexpression
• Target site mutation of PBPs
• Production of beta-lactamases
• Reduced permeability due to porin loss

Correct Answer: Production of beta-lactamases

Q11. Chloramphenicol inhibits bacterial protein synthesis by:

• Binding the 50S ribosomal subunit and inhibiting peptidyl transferase
• Blocking initiation at the 30S subunit
• Interfering with tRNA charging
• Preventing mRNA elongation by inhibiting RNA polymerase

Correct Answer: Binding the 50S ribosomal subunit and inhibiting peptidyl transferase

Q12. Linezolid’s mechanism of action involves:

• Inhibiting 50S ribosomal subunit formation by preventing initiation complex assembly
• Inhibiting DNA gyrase
• Binding ergosterol in fungal membranes
• Blocking dihydrofolate reductase

Correct Answer: Inhibiting 50S ribosomal subunit formation by preventing initiation complex assembly

Q13. Trimethoprim acts by inhibiting:

• Dihydropteroate synthase
• Dihydrofolate reductase
• DNA-dependent RNA polymerase
• Bacterial ribosomal peptidyl transferase

Correct Answer: Dihydrofolate reductase

Q14. Which PK/PD parameter best predicts efficacy for aminoglycosides?

• Time above MIC (T>MIC)
• AUC/MIC
• Cmax/MIC (peak concentration to MIC ratio)
• Post-antibiotic effect does not apply

Correct Answer: Cmax/MIC (peak concentration to MIC ratio)

Q15. Rifampicin inhibits bacterial growth by:

• Inhibiting DNA gyrase
• Binding to and inhibiting DNA-dependent RNA polymerase (β subunit)
• Blocking the 30S ribosomal subunit
• Disrupting peptidoglycan cross-linking

Correct Answer: Binding to and inhibiting DNA-dependent RNA polymerase (β subunit)

Q16. Echinocandins, such as caspofungin, act by:

• Inhibiting ergosterol synthesis via 14α-demethylase
• Binding to ergosterol forming pores
• Inhibiting β-(1,3)-D-glucan synthase, weakening fungal cell wall
• Intercalating fungal DNA and preventing replication

Correct Answer: Inhibiting β-(1,3)-D-glucan synthase, weakening fungal cell wall

Q17. Which mechanism explains bacterial resistance via modification of antibiotic target by methylation of 23S rRNA?

• Production of extended-spectrum beta-lactamases
• Target site modification causing macrolide resistance
• Enzymatic drug inactivation via acetylation
• Upregulation of porin channels

Correct Answer: Target site modification causing macrolide resistance

Q18. Metronidazole’s antimicrobial action requires activation by:

• Host hepatic enzymes only
• Anaerobic microbial reduction generating free radicals that damage DNA
• Binding to fungal ergosterol
• Direct inhibition of bacterial ribosomes

Correct Answer: Anaerobic microbial reduction generating free radicals that damage DNA

Q19. Which statement best describes bacteriostatic versus bactericidal action?

• Bacteriostatic agents kill bacteria; bactericidal agents only inhibit growth
• Bacteriostatic agents inhibit bacterial growth; bactericidal agents kill bacteria
• Only bacteriostatic agents require host immune system for eradication
• Bactericidal agents always act faster than bacteriostatic agents

Correct Answer: Bacteriostatic agents inhibit bacterial growth; bactericidal agents kill bacteria

Q20. Which antibiotic class is known to cause reversible inhibition of bacterial protein synthesis by binding the 30S subunit and blocking aminoacyl-tRNA binding?

• Tetracyclines
• Macrolides
• Glycopeptides
• Fluoroquinolones

Correct Answer: Tetracyclines

Q21. Polymyxins primarily act against Gram-negative bacteria by:

• Inhibiting cell wall synthesis
• Binding LPS and disrupting bacterial outer and inner membranes
• Inhibiting DNA gyrase
• Interfering with folate metabolism

Correct Answer: Binding LPS and disrupting bacterial outer and inner membranes

Q22. Which drug inhibits mycolic acid synthesis and is a first-line agent for tuberculosis?

• Isoniazid (INH)
• Rifampicin
• Ethambutol
• Pyrazinamide

Correct Answer: Isoniazid (INH)

Q23. Mechanism of action of azoles (e.g., fluconazole) involves:

• Binding ergosterol and forming pores
• Inhibition of 14α-demethylase, decreasing ergosterol synthesis
• Inhibition of β-(1,3)-D-glucan synthase
• Alkylation of fungal DNA

Correct Answer: Inhibition of 14α-demethylase, decreasing ergosterol synthesis

Q24. Which mechanism is most commonly responsible for macrolide resistance in streptococci?

• Efflux pump mediated by mef genes
• Enzymatic degradation by beta-lactamases
• Methylation of the 23S rRNA binding site
• Porin channel mutation

Correct Answer: Methylation of the 23S rRNA binding site

Q25. Bacteria with altered PBPs (e.g., MRSA) are resistant to many beta-lactams due to:

• Enzymatic cleavage of antibiotics
• Decreased drug influx through porins
• Acquisition of mecA gene encoding PBP2a with low affinity for beta-lactams
• Overexpression of dihydrofolate reductase

Correct Answer: Acquisition of mecA gene encoding PBP2a with low affinity for beta-lactams

Q26. Which statement about post-antibiotic effect (PAE) is correct?

• PAE is the immediate bactericidal effect during antibiotic exposure
• PAE is the persistent suppression of bacterial growth after brief antibiotic exposure
• PAE applies only to beta-lactam antibiotics
• PAE indicates antibiotic antagonism when combined with other drugs

Correct Answer: PAE is the persistent suppression of bacterial growth after brief antibiotic exposure

Q27. Which antimicrobial targets bacterial folate synthesis and is often combined with trimethoprim for synergy?

• Sulfonamides (e.g., sulfamethoxazole)
• Chloramphenicol
• Vancomycin
• Amphotericin B

Correct Answer: Sulfonamides (e.g., sulfamethoxazole)

Q28. Which of the following describes the mechanism of action of metronidazole against anaerobic bacteria?

• Competitive inhibition of dihydropteroate synthase
• Reduction by ferredoxin in anaerobes to form free radicals that damage DNA
• Binding to 50S ribosomal subunit and preventing translocation
• Inhibition of peptidoglycan transglycosylase

Correct Answer: Reduction by ferredoxin in anaerobes to form free radicals that damage DNA

Q29. Which parameter defines the minimal concentration of drug that inhibits visible growth of bacteria?

• MBC (Minimum Bactericidal Concentration)
• MIC (Minimum Inhibitory Concentration)
• EC50 (Effective Concentration 50%)
• LD50 (Lethal Dose 50%)

Correct Answer: MIC (Minimum Inhibitory Concentration)

Q30. Which drug class inhibits the 50S ribosomal subunit by preventing formation of peptide bonds and can cause aplastic anemia?

• Macrolides
• Chloramphenicol
• Tetracyclines
• Aminoglycosides

Correct Answer: Chloramphenicol

Q31. Which mechanism is used by bacteria to resist aminoglycosides?

• Methylation of 23S rRNA
• Enzymatic modification of the antibiotic (acetylation, phosphorylation, adenylation)
• Overproduction of peptidoglycan
• Increased ergosterol synthesis

Correct Answer: Enzymatic modification of the antibiotic (acetylation, phosphorylation, adenylation)

Q32. Which of the following antiviral agents inhibits influenza neuraminidase, preventing viral release?

• Acyclovir
• Oseltamivir
• Zidovudine
• Ribavirin

Correct Answer: Oseltamivir

Q33. Which antibiotic’s efficacy is best correlated with time above the MIC (T>MIC)?

• Aminoglycosides
• Fluoroquinolones
• Beta-lactams
• Glycopeptides

Correct Answer: Beta-lactams

Q34. Which mechanism describes how sulfonamide resistance often occurs in bacteria?

• Mutation in dihydropteroate synthase decreasing drug binding
• Increased drug uptake
• Conversion of drug to active metabolite
• Overactivation of host immune response

Correct Answer: Mutation in dihydropteroate synthase decreasing drug binding

Q35. Which drug is a cell wall synthesis inhibitor that specifically inhibits arabinosyl transferase in mycobacteria?

• Ethambutol
• Isoniazid
• Rifampicin
• Pyrazinamide

Correct Answer: Ethambutol

Q36. Which mechanism explains how integrase inhibitors act against HIV?

• Blocking reverse transcriptase activity
• Inhibiting HIV protease maturation step
• Preventing integration of viral DNA into host genome by inhibiting integrase
• Blocking viral entry by binding CD4 receptors

Correct Answer: Preventing integration of viral DNA into host genome by inhibiting integrase

Q37. Which of the following best describes antagonism between bacteriostatic and bactericidal antibiotics?

• Bacteriostatic agents enhance bactericidal killing always
• Bacteriostatic agents may antagonize bactericidal drugs that require active bacterial growth
• Antagonism only occurs with antifungal combinations
• Antagonism means two drugs always produce more effect together

Correct Answer: Bacteriostatic agents may antagonize bactericidal drugs that require active bacterial growth

Q38. Which mechanism is characteristic of mupirocin’s antibacterial action?

• Inhibition of isoleucyl-tRNA synthetase, preventing protein synthesis
• Disrupting cell membrane by binding ergosterol
• Inhibiting DNA gyrase
• Blocking peptidoglycan cross-linking by binding D-Ala-D-Ala

Correct Answer: Inhibition of isoleucyl-tRNA synthetase, preventing protein synthesis

Q39. Which drug class is known for causing tendonitis and tendon rupture due to effects on connective tissue, linked to its mechanism?

• Fluoroquinolones
• Macrolides
• Tetracyclines
• Penicillins

Correct Answer: Fluoroquinolones

Q40. Which of the following best explains how rifampicin resistance commonly arises?

• Overexpression of efflux pumps only
• Mutation in the rpoB gene encoding the β subunit of RNA polymerase
• Production of beta-lactamases
• Loss of porin channels in Gram-negative bacteria

Correct Answer: Mutation in the rpoB gene encoding the β subunit of RNA polymerase

Q41. Which antifungal inhibits squalene epoxidase, decreasing ergosterol synthesis and used for dermatophytes?

• Amphotericin B
• Terbinafine
• Fluconazole
• Caspofungin

Correct Answer: Terbinafine

Q42. Which mechanism explains how beta-lactamase inhibitors (e.g., clavulanic acid) restore beta-lactam activity?

• They bind PBPs to enhance antibiotic binding
• They irreversibly inhibit beta-lactamase enzymes, preventing antibiotic degradation
• They increase bacterial porin expression
• They block efflux pumps

Correct Answer: They irreversibly inhibit beta-lactamase enzymes, preventing antibiotic degradation

Q43. Which measure indicates the lowest concentration of antibiotic that kills 99.9% of the initial inoculum?

• MIC
• MBC
• PAE
• EC90

Correct Answer: MBC

Q44. Which antibiotic binds the 30S ribosomal subunit and causes misreading of mRNA, requiring oxygen-dependent uptake into bacteria?

• Aminoglycosides (e.g., gentamicin)
• Tetracyclines
• Macrolides
• Glycopeptides

Correct Answer: Aminoglycosides (e.g., gentamicin)

Q45. Which mechanism is associated with vancomycin resistance in Enterococci (VRE)?

• Production of beta-lactamases
• Substitution of D-Ala-D-Ala with D-Ala-D-Lac reducing vancomycin binding
• Increased ergosterol content in cell membrane
• Mutation in 23S rRNA

Correct Answer: Substitution of D-Ala-D-Ala with D-Ala-D-Lac reducing vancomycin binding

Q46. Which antibiotic is a prodrug activated by bacterial nitroreductases and used against Helicobacter pylori?

• Metronidazole
• Clarithromycin
• Amoxicillin
• Levofloxacin

Correct Answer: Metronidazole

Q47. Which mechanism explains how bacterial efflux pumps confer multidrug resistance?

• By enzymatically modifying antibiotics to inactive forms
• By actively transporting diverse antibiotics out of the cell, lowering intracellular concentration
• By preventing antibiotic absorption in the gut
• By increasing DNA repair mechanisms

Correct Answer: By actively transporting diverse antibiotics out of the cell, lowering intracellular concentration

Q48. Which antitubercular drug inhibits mycolic acid synthesis by targeting enoyl-ACP reductase (InhA) after activation by KatG?

• Ethambutol
• Isoniazid
• Rifampicin
• Streptomycin

Correct Answer: Isoniazid

Q49. Which antiviral drug class inhibits HIV protease, preventing maturation of viral particles?

• Nucleoside reverse transcriptase inhibitors (NRTIs)
• Protease inhibitors (PIs)
• Integrase inhibitors
• Fusion inhibitors

Correct Answer: Protease inhibitors (PIs)

Q50. Which principle explains why time-dependent antibiotics require maintaining drug concentrations above MIC for optimal efficacy?

• Their bactericidal action correlates with peak concentration only
• Their killing increases with duration of exposure rather than peak levels, so T>MIC predicts efficacy
• They have no post-antibiotic effect
• They act only on dormant bacteria

Correct Answer: Their killing increases with duration of exposure rather than peak levels, so T>MIC predicts efficacy

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com