**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). This mechanism of action is unique to beta-lactam antibiotics, including penicillins, cephalosporins, and monobactams.

**Why the Correct Answer is Right**
Penicillin G is a water-soluble antibiotic that is effective against Gram-positive cocci, including Streptococcus pneumoniae and Staphylococcus aureus. It is also used to treat syphilis, caused by the bacterium Treponema pallidum. Penicillin G is administered parenterally, as it is rapidly destroyed by oral enzymes.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect, as penicillin G is not effective against Pseudomonas aeruginosa, a Gram-negative bacterium that is often resistant to beta-lactam antibiotics.
**Option B:** This statement is incorrect, as penicillin G is not a broad-spectrum antibiotic. It is typically used to treat infections caused by Gram-positive bacteria.
**Option C:** This statement is incorrect, as penicillin G is not a prodrug that requires activation by the liver. It is administered in its active form.

**Clinical Pearl / High-Yield Fact**
Penicillin G is a classic example of a narrow-spectrum antibiotic, and its effectiveness is limited to Gram-positive bacteria. This is why it is often used in combination with other antibiotics to treat infections caused by Gram-negative bacteria.

**Correct Answer: B. Penicillin G is a broad-spectrum antibiotic.**