Which of the following drugs is NOT a penicillin?

Azithromycin is the correct answer because it belongs to a different class of antibiotics known as macrolides, rather than the penicillin class. Macrolides work by inhibiting bacterial protein synthesis, while penicillins, such as amoxicillin, cloxacillin, and nafcillin sodium, are beta-lactam antibiotics that target bacterial cell wall synthesis. Hence, while amoxicillin, cloxacillin, and nafcillin sodium all share the penicillin structure and properties, azithromycin does not, making it distinct from the other options in the question.