Which antibiotic is a macrolide?

• A. Chloramphenicol
• B. Erythromycin
• C. Tetracycline
• D. Vancomycin

Answer: (B)

Macrolides are antibiotics defined by a large macrocyclic lactone ring and by binding to the 50S ribosomal subunit to block the translocation step of protein synthesis. Erythromycin is the classic example of this class, so it fits as the macrolide here. Its action prevents the movement of peptidyl-tRNA from the A site to the P site on the ribosome, stopping peptide elongation and thereby inhibiting bacterial protein synthesis. Other options work by different mechanisms or belong to different classes: tetracycline targets the 30S subunit and blocks tRNA binding, vancomycin inhibits cell wall synthesis, and chloramphenicol inhibits peptidyl transferase but is not a macrolide.