Which of the following antibiotics is considered a penicillinase-resistant penicillin?

Nafcillin is considered a penicillinase-resistant penicillin because it was specifically developed to counteract the enzymatic activity of beta-lactamases, which are enzymes produced by certain bacteria that can inactivate penicillins and other beta-lactam antibiotics. By being resistant to these enzymes, nafcillin remains effective against penicillin-sensitive strains of bacteria that produce penicillinase.

This characteristic makes nafcillin particularly useful in treating infections caused by penicillinase-producing staphylococcus species, which are known to be resistant to other forms of penicillin such as amoxicillin. In contrast, the other options either do not possess this specific resistance mechanism or belong to different antibiotic classes, thereby not fitting the criteria for penicillinase-resistant penicillins. For instance, amoxicillin is susceptible to penicillinase, azithromycin is a macrolide antibiotic not related to penicillins, and cefalexin is a first-generation cephalosporin, which, while offering similar efficacy against some bacteria, does not have the same profile of resistance to penicillinase.