β-Lactamase-stable penicillins: Which penicillin listed is formulated to resist staphylococcal β-lactamases and thus remain active against many penicillinase-producing strains?

Introduction / Context:
Some penicillins are structurally modified to resist hydrolysis by staphylococcal β-lactamases. Recognizing these agents helps target methicillin-susceptible Staphylococcus aureus (MSSA).

Given Data / Assumptions:

• Cloxacillin/oxacillin/nafcillin are penicillinase-resistant antistaphylococcal penicillins.
• Other penicillins are hydrolyzed by common β-lactamases.
• The option spelling ‘‘closacillin’’ corresponds to cloxacillin.

Concept / Approach:
Bulky side chains in antistaphylococcal penicillins hinder access of β-lactamases to the β-lactam ring. They are preferred for MSSA bacteremia and deep infections. In contrast, ampicillin, carbenicillin, and piperacillin are extended-spectrum agents but remain β-lactamase-susceptible unless paired with inhibitors.

Step-by-Step Solution:
Identify the β-lactamase-stable choice → cloxacillin.Exclude penicillin G (labile) and aminopenicillins/ureidopenicillins without inhibitors.Select option ‘‘closacillin (cloxacillin)’’.

Verification / Alternative check:
Clinical guidelines recommend nafcillin/oxacillin/flucloxacillin for MSSA, aligning with cloxacillin’s profile.

Why Other Options Are Wrong:
They are either penicillinase-labile or require β-lactamase inhibitors to regain activity.

Common Pitfalls:
Confusing activity against Pseudomonas (piperacillin) with β-lactamase stability; these are separate properties.

Final Answer:
closacillin (cloxacillin).