β-lactamase-resistant penicillins — Identify the penicillin that is NOT resistant to staphylococcal β-lactamase.

Introduction / Context:Staphylococcal penicillinase (a β-lactamase) hydrolyzes many penicillins. Certain agents (e.g., oxacillin, nafcillin, methicillin) are engineered to resist this enzyme, making them effective for methicillin-susceptible S. aureus (MSSA).

Given Data / Assumptions:

• Oxacillin, nafcillin, methicillin: classic β-lactamase–resistant antistaphylococcal penicillins.
• Carbenicillin: extended-spectrum carboxypenicillin active against Pseudomonas, but β-lactamase-susceptible.

Concept / Approach:Select the option that lacks β-lactamase stability against staphylococcal enzymes. Carbenicillin does not have the steric features that protect the β-lactam ring from penicillinase.

Step-by-Step Solution:

Verification / Alternative check:Therapeutic guidelines use oxacillin/nafcillin (or cefazolin) for MSSA; carbenicillin is not used for that purpose.

Why Other Options Are Wrong:

• Oxacillin, nafcillin, methicillin: designed to resist staphylococcal penicillinase.

Common Pitfalls:Assuming broad spectrum (e.g., antipseudomonal) implies penicillinase resistance; spectrum and enzyme stability are distinct properties.

Final Answer:Carbenicillin