**Core Concept**
The question tests knowledge of antimicrobial therapy, specifically the treatment of infections caused by gram-negative anaerobes. **Anaerobic bacteria** are organisms that do not require oxygen for growth and can cause various infections.

**Why the Correct Answer is Right**
The correct answer is effective against common gram-negative anaerobes due to its mechanism of action, which involves inhibiting cell wall synthesis or protein synthesis in bacteria. This is crucial for treating infections caused by **Bacteroides** species, a common type of gram-negative anaerobe.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not typically used for gram-negative anaerobes.
**Option B:** Incorrect as it has limited coverage against anaerobes.
**Option C:** Not the best choice due to resistance patterns.

**Clinical Pearl / High-Yield Fact**
Metronidazole is a key drug for treating anaerobic infections, particularly those caused by **gram-negative anaerobes** like **Bacteroides fragilis**.

**Correct Answer:** D. Metronidazole