Penicillin targets peptidoglycan in bacterial cell walls

Steps:

• Identify penicillin's mechanism: it inhibits transpeptidase, blocking peptidoglycan cross-linking essential for bacterial cell walls.
• Recall organisms with peptidoglycan: only bacteria (both Gram-positive and Gram-negative).
• Determine non-targets: fungi have chitin walls, archaea have pseudopeptidoglycan, viruses lack cell walls.
• Match to choices: assuming 1 is bacteria, 2 is fungi/archaea, 3 is viruses/eukaryotes, only 1 is affected.

Why C is correct:

• Penicillin specifically disrupts peptidoglycan synthesis, a unique bacterial cell wall component (definition from antibiotic mechanism).

Why the others are wrong:

• A includes 2, but non-bacterial organisms lack peptidoglycan.
• B includes 3, but viruses/eukaryotes have no peptidoglycan walls.
• D excludes 1 but includes 2 and 3, ignoring bacteria as the sole target.

Final answer: C