Penicillins vs. non-penicillins: Which of the following is NOT a penicillin-class antibiotic (choose the non-penicillin)?

Introduction / Context:
Beta-lactam antibiotics include several subclasses with related mechanisms but distinct core structures: penicillins, cephalosporins, carbapenems, and monobactams. Distinguishing penicillins from non-penicillins is a foundational pharmacy and microbiology skill.

Given Data / Assumptions (Recovery Note):

• The original options included ambiguous names (e.g., “Penicillin X/Y”), which are not standard in clinical use. Applying the Recovery-First Policy, we reconstruct realistic choices while preserving the intended concept: identify a non-penicillin among common antibiotics.
• We include two canonical penicillins (G and V) and a widely used aminopenicillin (ampicillin) alongside a cephalosporin (cephalexin).

Concept / Approach:
Penicillins share a beta-lactam fused to a thiazolidine ring (penam nucleus). Cephalosporins have a beta-lactam fused to a dihydrothiazine ring (cephem nucleus). While both inhibit transpeptidases involved in peptidoglycan synthesis, they are distinct classes.

Step-by-Step Solution:

Verification / Alternative check:
Drug references classify cephalexin under cephalosporins; penicillin G/V and ampicillin are well-known penicillins.

Why Other Options Are Wrong:

• Penicillin G: a natural penicillin used parenterally.
• Penicillin V: an orally stable natural penicillin.
• Ampicillin: a semisynthetic penicillin with broader Gram-negative activity.

Common Pitfalls:
Assuming all beta-lactams are “penicillins”; class matters for spectrum, stability, and allergy cross-reactivity.

Final Answer:
Cephalexin