Which antimicrobial agent acts by inhibiting protein synthesis?

• A. Gentamicin
• B. Methicillin
• C. Rifampin
• D. Ampicillin

Answer: (A)

Antibiotics that inhibit protein synthesis attack the bacterial ribosome, either the 30S or 50S subunits, to stop making essential proteins. Gentamicin is an aminoglycoside that binds the 30S ribosomal subunit, causing misreading of mRNA and blocking the initiation of protein synthesis, which is typically bactericidal. The other agents work differently: methicillin and ampicillin are beta-lactams that inhibit cell wall synthesis by blocking penicillin‑binding proteins, weakening the wall rather than the protein-making machinery; rifampin inhibits transcription by binding to RNA polymerase, preventing RNA synthesis rather than protein production. So gentamicin best fits the action of inhibiting protein synthesis.