Which Penicillin is NOT penicillinase resistant?

• A. Dicloxacillin
• B. Amoxicillin
• C. Augmentin
• D. Penicillin G

Answer: (B)

Penicillinase resistance means the antibiotic can withstand being broken down by beta-lactamase enzymes produced by some bacteria. Molecules with bulky side chains block the enzyme, giving true penicillinase resistance (like dicloxacillin). The classic penicillin, Penicillin G, is not resistant and is readily hydrolyzed by penicillinase. Amoxicillin itself is not resistant to penicillinase, so it’s degraded unless protected. Augmentin pairs amoxicillin with clavulanic acid, a beta-lactamase inhibitor, which protects the antibiotic from penicillinase—so the combination works where penicillin alone would be inactivated. When the question focuses on the intrinsic resistance of the penicillin molecule, amoxicillin is not penicillinase resistant, making it the correct choice.