**Core Concept**
Penicillin is a narrow-spectrum beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs). This action prevents the formation of a strong peptidoglycan layer in the bacterial cell wall, ultimately leading to cell lysis and death.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of action of penicillin. Penicillin works by inhibiting the transpeptidase reaction, which is essential for the cross-linking of peptidoglycan chains. This is achieved by binding to PBPs, enzymes responsible for the final steps of peptidoglycan synthesis. By inhibiting these enzymes, penicillin disrupts the structural integrity of the bacterial cell wall, ultimately leading to cell death. The correct answer is related to this mechanism of action.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe the correct mechanism of action of penicillin. While penicillin does inhibit protein synthesis, this is not its primary mechanism of action.

**Option B:** This option is incorrect because it does not accurately describe the spectrum of activity of penicillin. Penicillin is a narrow-spectrum antibiotic, effective against certain Gram-positive and Gram-negative bacteria, but not all.

**Option C:** This option is incorrect because it does not describe the correct target of penicillin. Penicillin targets PBPs, not DNA replication or transcription.

**Option D:** This option is incorrect because it does not accurately describe the primary mechanism of action of penicillin. While penicillin can affect cell membrane function, this is not its primary mechanism of action.

**Clinical Pearl / High-Yield Fact**
To remember the mechanism of action of penicillin, recall that it inhibits the cross-linking of peptidoglycan chains by binding to PBPs. This is a critical step in the synthesis of a strong bacterial cell wall.

**Correct Answer:** B. Penicillin is effective against Gram-positive bacteria.