Which antimicrobial works by inhibiting cell-wall synthesis?

• A. Ertapenem
• B. Azithromycin
• C. Doxycycline
• D. Ciprofloxacin

Answer: (A)

Recognizing the mechanism of action: some antibiotics kill bacteria by preventing the building of the cell wall. Ertapenem is a carbapenem that belongs to the beta-lactam family and binds to penicillin-binding proteins, blocking the cross-linking of peptidoglycan in the cell wall. Without a proper cell wall, actively growing bacteria cannot maintain their integrity and burst, making this class specifically about cell-wall synthesis inhibition.

In contrast, azithromycin binds to the 50S ribosomal subunit to halt protein synthesis, doxycycline blocks the 30S subunit to stop protein synthesis, and ciprofloxacin inhibits DNA gyrase and topoisomerase IV, hindering DNA replication. These mechanisms are not about cell-wall synthesis, which is why ertapenem is the correct choice.