Which of the following agents is not an example of an extended-spectrum penicillin?

• A. Amoxicillin
• B. Piperacillin
• C. Ticarcillin
• D. Nafcillin

Answer: (D)

The basic idea is that extended-spectrum penicillins broaden coverage to more Gram-negative bacteria, making them useful against a wider range of infections. Nafcillin is not in this group; it’s a penicillinase-resistant (anti-staphylococcal) penicillin, designed to resist staphylococcal beta-lactamases and focus on Gram-positive organisms. Its spectrum is comparatively narrower for Gram-negatives, so it’s not considered extended-spectrum. Amoxicillin, piperacillin, and ticarcillin all have expanded activity against a broader set of bacteria, including many Gram-negatives (with piperacillin and ticarcillin offering notable anti-pseudomonal activity), though they can be inactivated by beta-lactamases unless used with inhibitors. So the agent that is not an extended-spectrum penicillin is nafcilin.