Which of the following penicillins is NOT a type of penicillin?

Cefalexin is the correct answer because it is not a type of penicillin; rather, it belongs to the class of antibiotics known as cephalosporins. Cephalosporins share a similar beta-lactam structure with penicillins, but they have different chemical properties and mechanisms of action, making them a distinct group of antibiotics used to treat infections.

In contrast, the other drugs listed—amoxicillin, benzylpenicillin, and flucloxacillin—are all types of penicillins. Amoxicillin is a broad-spectrum penicillin commonly used in various infections, benzylpenicillin (also known as penicillin G) is often utilized for serious infections, and flucloxacillin is specifically effective against methicillin-resistant Staphylococcus aureus (MRSA) and is used in skin and soft tissue infections. Understanding these categories helps in selecting the appropriate antibiotic based on the infection being treated.