β-Lactam antibiotics – classification and mechanism MCQs With Answer

Introduction

β-Lactam antibiotics are a cornerstone of antimicrobial therapy for B.Pharm students to master. This group — including penicillins, cephalosporins, carbapenems and monobactams — is defined by the β-lactam ring that targets bacterial cell wall synthesis by binding penicillin‑binding proteins (PBPs) and inhibiting transpeptidation. Key concepts include classification by spectrum and generation, mechanisms of action and resistance (β‑lactamases, altered PBPs, porin changes), pharmacokinetics, clinical uses, adverse effects and β‑lactamase inhibitors (e.g., clavulanic acid). Understanding time‑dependent killing, MIC, and structural modifications is essential. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which structural feature is essential for the antibacterial activity of β‑lactam antibiotics?

• The thiazolidine ring
• The β‑lactam ring
• Amino sugar side chain
• Benzyl penicillin side chain

Correct Answer: The β‑lactam ring

Q2. The primary molecular target of β‑lactam antibiotics in bacteria is:

• DNA gyrase
• Ribosomal 30S subunit
• Penicillin‑binding proteins (PBPs)
• Topoisomerase IV

Correct Answer: Penicillin‑binding proteins (PBPs)

Q3. Inhibition of transpeptidation by β‑lactams leads directly to:

• Inhibition of protein synthesis
• Impaired peptidoglycan cross‑linking
• Blockade of folate metabolism
• Disruption of nucleic acid synthesis

Correct Answer: Impaired peptidoglycan cross‑linking

Q4. Which enzyme produced by bacteria hydrolyzes the β‑lactam ring and confers resistance?

• Transpeptidase
• β‑Lactamase
• DNA polymerase
• Acetyltransferase

Correct Answer: β‑Lactamase

Q5. Which β‑lactam subclass is known for broadest spectrum against Gram‑negative rods including Pseudomonas and has high β‑lactamase stability?

• First‑generation cephalosporins
• Monobactams
• Carbapenems
• Natural penicillins

Correct Answer: Carbapenems

Q6. Aztreonam belongs to which β‑lactam class and is especially useful for patients allergic to penicillin?

• Carbapenem
• Monobactam
• First‑generation cephalosporin
• Penicillinase‑resistant penicillin

Correct Answer: Monobactam

Q7. Clavulanic acid is combined with amoxicillin primarily to:

• Enhance renal clearance
• Extend half‑life by protein binding
• Inhibit bacterial β‑lactamases
• Reduce gastrointestinal side effects

Correct Answer: Inhibit bacterial β‑lactamases

Q8. Extended‑spectrum β‑lactamases (ESBLs) typically hydrolyze which β‑lactam antibiotics?

• Carbapenems only
• Penicillins and cephalosporins, especially third generation
• Monobactams exclusively
• Glycopeptides

Correct Answer: Penicillins and cephalosporins, especially third generation

Q9. Which pharmacodynamic property best describes β‑lactam antibiotics?

• Concentration‑dependent killing (Cmax/MIC)
• Time‑dependent killing (%T>MIC)
• Post‑antibiotic effect with AUC/MIC dependence
• Single‑dose eradication independent of time

Correct Answer: Time‑dependent killing (%T>MIC)

Q10. Methicillin resistance in Staphylococcus aureus (MRSA) is mediated by:

• Overproduction of β‑lactamase only
• Mutation causing altered PBPs (PBP2a encoded by mecA)
• Efflux pump increase
• Loss of porins

Correct Answer: Mutation causing altered PBPs (PBP2a encoded by mecA)

Q11. Which cephalosporin generation has improved activity against Gram‑negative organisms and some β‑lactamase stability?

• First generation
• Second generation
• Third generation
• Fourth generation

Correct Answer: Fourth generation

Q12. Probenecid is sometimes used with penicillins because it:

• Inhibits β‑lactamase enzymes
• Decreases renal tubular secretion of penicillins, prolonging plasma levels
• Enhances bacterial uptake
• Prevents allergic reactions

Correct Answer: Decreases renal tubular secretion of penicillins, prolonging plasma levels

Q13. Which adverse effect is most characteristically associated with β‑lactam antibiotics?

• Ototoxicity
• Nephrotoxicity via acute interstitial nephritis and allergic nephritis
• Gray baby syndrome
• Tendon rupture

Correct Answer: Nephrotoxicity via acute interstitial nephritis and allergic nephritis

Q14. The ability of β‑lactams to activate bacterial autolysins leads to:

• Increased protein synthesis
• Cell wall degradation and cell lysis
• Inhibition of DNA replication
• Prevention of spore formation

Correct Answer: Cell wall degradation and cell lysis

Q15. Which modification increases oral bioavailability of a penicillin?

• Addition of bulky, acid‑sensitive side chain
• Making the molecule more susceptible to gastric acid
• Inclusion of an acid‑stable side chain such as in amoxicillin
• Conversion to a parenteral-only salt form

Correct Answer: Inclusion of an acid‑stable side chain such as in amoxicillin

Q16. Which β‑lactam is least likely to cause cross‑reactive allergic reactions in a penicillin‑allergic patient?

• Amoxicillin
• Cefazolin
• Aztreonam
• Oxacillin

Correct Answer: Aztreonam

Q17. Which mechanism is NOT a common bacterial resistance mechanism to β‑lactams?

• Production of β‑lactamase enzymes
• Alteration of PBPs
• Active efflux of β‑lactam into periplasmic space to concentrate drug
• Decreased permeability via porin loss

Correct Answer: Active efflux of β‑lactam into periplasmic space to concentrate drug

Q18. Which laboratory parameter best quantifies the potency of a β‑lactam against a bacterial strain?

• Minimum inhibitory concentration (MIC)
• Therapeutic index
• Serum half‑life
• Volume of distribution

Correct Answer: Minimum inhibitory concentration (MIC)

Q19. Piperacillin is often combined with tazobactam primarily to:

• Reduce dosing frequency
• Enhance penetration into Gram‑positive cocci
• Inhibit β‑lactamases and broaden spectrum
• Prevent nephrotoxicity

Correct Answer: Inhibit β‑lactamases and broaden spectrum

Q20. Which statement about cephalosporin generations is correct?

• First‑generation cephalosporins have the best activity against Pseudomonas
• Third‑generation cephalosporins generally have improved CNS penetration and Gram‑negative coverage
• Second‑generation cephalosporins target anaerobes exclusively
• Fourth‑generation cephalosporins are less stable to β‑lactamases than first generation

Correct Answer: Third‑generation cephalosporins generally have improved CNS penetration and Gram‑negative coverage

Q21. Which β‑lactam antibiotic class lacks cross‑reactivity with penicillins due to different ring structure?

• Cephalosporins
• Carbapenems
• Monobactams
• Penicillinase‑resistant penicillins

Correct Answer: Monobactams

Q22. A β‑lactam that is particularly resistant to staphylococcal β‑lactamase is:

• Penicillin G
• Methicillin/oxacillin (penicillinase‑resistant penicillins)
• Amoxicillin
• Ceftriaxone

Correct Answer: Methicillin/oxacillin (penicillinase‑resistant penicillins)

Q23. Which clinical use best matches imipenem combined with cilastatin?

• Oral therapy for mild urinary tract infections
• Broad‑spectrum parenteral therapy for severe mixed infections; cilastatin inhibits renal dehydropeptidase‑I to prevent imipenem degradation
• Topical treatment of skin infections only
• Long‑term prophylaxis in endocarditis

Correct Answer: Broad‑spectrum parenteral therapy for severe mixed infections; cilastatin inhibits renal dehydropeptidase‑I to prevent imipenem degradation

Q24. Beta‑lactam antibiotics are bactericidal primarily because they:

• Inhibit folic acid synthesis
• Interfere with cell wall synthesis leading to osmotic lysis
• Inhibit 50S ribosomal subunit
• Disrupt bacterial membranes like polymyxins

Correct Answer: Interfere with cell wall synthesis leading to osmotic lysis

Q25. A common laboratory method to detect β‑lactamase production is:

• Gram staining
• Nitrocefin test
• Coagulase test
• Oxidase test

Correct Answer: Nitrocefin test

Q26. Which factor decreases the efficacy of a β‑lactam antibiotic against an intracellular pathogen?

• Excellent lipid solubility and intracellular penetration
• Time‑dependent killing kinetics
• Poor penetration into host cells and low intracellular concentrations
• High affinity for bacterial PBPs

Correct Answer: Poor penetration into host cells and low intracellular concentrations

Q27. Which β‑lactam is recommended for anaerobic infections such as Bacteroides fragilis when combined with a β‑lactamase inhibitor?

• Amoxicillin alone
• Piperacillin‑tazobactam
• Oxacillin alone
• Cefazolin alone

Correct Answer: Piperacillin‑tazobactam

Q28. Which structural change in PBPs confers resistance to β‑lactams?

• Increased expression of native PBPs without mutation
• Point mutation in PBP reducing β‑lactam affinity (e.g., PBP2a)
• Complete loss of PBPs
• Increased porin channels to efflux drug

Correct Answer: Point mutation in PBP reducing β‑lactam affinity (e.g., PBP2a)

Q29. The post‑antibiotic effect (PAE) for β‑lactams is generally:

• Prolonged even with short exposures and is the main driver of once‑daily dosing
• Minimal or short; dosing frequency depends on maintaining %T>MIC
• Independent of bacterial species
• So large that therapeutic levels are unnecessary

Correct Answer: Minimal or short; dosing frequency depends on maintaining %T>MIC

Q30. Which laboratory susceptibility parameter guides time‑dependent β‑lactam dosing decisions?

• Peak serum concentration (Cmax)
• % Time above MIC (%T>MIC)
• Area under the curve to MIC ratio (AUC/MIC) only
• Protein binding percentage

Correct Answer: % Time above MIC (%T>MIC)

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