Which antibiotic is a macrolide?

• A. Erythromycin
• B. Clindamycin
• C. Amoxicillin
• D. Ceftriaxone

Answer: (A)

Macrolides inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit and blocking the translocation step, which stops the growing peptide chain. Erythromycin is the classic example of a macrolide, illustrating this mechanism. This class is typically effective against many Gram-positive cocci and atypical pathogens such as Mycoplasma and Legionella, making it a common choice for respiratory infections. Clindamycin also targets the 50S subunit but is a lincosamide, not a macrolide. Amoxicillin and ceftriaxone are beta-lactams that inhibit cell wall synthesis, not protein synthesis, so they are not macrolides.