Which penicillin is the penicillinase-resistant option?

• A. Amoxicillin
• B. Benzylpenicillin
• C. Cloxacillin
• D. Ampicillin

Answer: (C)

Penicillinase resistance comes from having a bulky side chain that blocks the enzyme beta-lactamase from opening the beta-lactam ring. Drugs with this isoxazolyl side chain, like cloxacillin, are stable against penicillinase produced by some Staphylococcus aureus. The other commonly used penicillins, such as amoxicillin or ampicillin, are susceptible to beta-lactamase unless paired with an inhibitor, and benzylpenicillin (penicillin G) is also inactivated by penicillinase. So cloxacillin is the penicillinase-resistant option.