The drugs Azithromycin, Clarithromycin, Erythromycin, and Fidaxomicin belong to which class?

• A. Macrolides
• B. Penicillins
• C. Aminoglycosides
• D. Tetracyclines

Answer: (A)

Macrolides are a group of antibiotics built around a large lactone ring. They work by binding to the 50S ribosomal subunit and blocking the translocation step of protein synthesis, which slows bacterial growth. Azithromycin, clarithromycin, and erythromycin are classic examples of this class, sharing similar targets and uses such as respiratory and atypical infections. Fidaxomicin is a newer macrocyclic antibiotic that fits this group in structure and clinical use, particularly for Clostridioides difficile infections, which is why it’s included with the macrolides.

Other commonly tested classes differ in mechanism and targets: penicillins are beta-lactams that disrupt cell wall synthesis; aminoglycosides target the 30S subunit and are usually bactericidal; tetracyclines also target the 30S but are typically bacteriostatic.