**Core Concept**
Penicillin G, a narrow-spectrum beta-lactam antibiotic, is used to treat various bacterial infections. Its mechanism of action involves inhibiting cell wall synthesis by binding to penicillin-binding proteins (PBPs), ultimately leading to bacterial cell lysis and death.

**Why the Correct Answer is Right**
Penicillin G is primarily used to treat infections caused by susceptible gram-positive and some gram-negative bacteria, including streptococci, staphylococci, and pneumococci. Its efficacy against these pathogens is due to its ability to cross the bacterial cell membrane and bind to PBPs, thereby inhibiting the synthesis of the bacterial cell wall. This leads to the accumulation of peptidoglycan precursors, causing the cell wall to become weakened and ultimately leading to bacterial cell lysis.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because penicillin G is effective against various gram-positive and some gram-negative bacteria, making it a suitable choice for treating infections caused by these pathogens.

**Option B:** Incorrect because penicillin G is not typically used to treat methicillin-resistant Staphylococcus aureus (MRSA) infections, as MRSA has developed resistance to methicillin and other penicillins.

**Option C:** Incorrect because penicillin G is not effective against most gram-negative bacteria, which have an outer membrane that prevents the antibiotic from reaching the PBPs.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that penicillin G is not effective against beta-lactamase-producing bacteria, such as some gram-negative bacteria, as these enzymes can inactivate the antibiotic.

**Correct Answer:** B.