**Core Concept**
Penicillin G, also known as benzylpenicillin, is a narrow-spectrum beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs) on the bacterial cell wall. This action results in the weakening and eventual lysis of the bacterial cell wall, leading to bacterial cell death.

**Why the Correct Answer is Right**
Penicillin G is commonly used to treat infections caused by susceptible bacteria, such as streptococci, pneumococci, and meningococci. Its mechanism of action involves the inhibition of PBPs, which are essential for the cross-linking of peptidoglycan layers in the bacterial cell wall. This results in the accumulation of peptidoglycan precursors and the eventual lysis of the bacterial cell wall.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because penicillin G is indeed used to treat streptococcal infections, such as strep throat and skin infections.
**Option B:** This option is incorrect because penicillin G is used to treat pneumococcal infections, including pneumonia and meningitis.
**Option C:** This option is incorrect because penicillin G is used to treat meningococcal infections, including meningitis.

**Clinical Pearl / High-Yield Fact**
Penicillin G is generally not effective against gram-negative bacteria, which are characterized by their thin peptidoglycan layer and outer membrane containing lipopolysaccharides. This is because gram-negative bacteria lack the PBPs that penicillin G targets.

**Correct Answer: D.**