Penicillin targets bacterial cell wall synthesis

Steps:

• Identify that penicillin kills bacteria by inhibiting peptidoglycan cross-linking in cell walls.
• Recall bacterial cells contain peptidoglycan (structure 1), penicillin-binding proteins (structure 2), and active transpeptidases (structure 4).
• Confirm eukaryotic cells lack these, so only bacterial cells are affected.
• Match to choices: A includes all relevant bacterial targets (1, 2, 4).

Why A is correct:

• Penicillin binds irreversibly to penicillin-binding proteins, preventing peptidoglycan synthesis essential for bacterial survival (definition of beta-lactam antibiotic action).

Why the others are wrong:

• B omits structure 4 (transpeptidases), incomplete for full mechanism.
• C omits structure 2 (binding proteins), missing key enzyme target.
• D includes only structure 4, ignoring peptidoglycan and proteins as primary sites.

Final answer: A