**Core Concept**
The nephrotoxicity of certain drugs represents a significant clinical concern, often necessitating careful selection and monitoring in patients with pre-existing renal impairment. This phenomenon is closely related to the drugs' ability to cause direct renal damage, induce renal vasoconstriction, or impair renal function through various mechanistic pathways.

**Why the Correct Answer is Right**
Aminoglycosides, such as Gentamicin (A), are well-known for their potential to cause nephrotoxicity. This effect is primarily attributed to the accumulation of the drug and its metabolites in renal tubular cells, leading to acute tubular necrosis. The mechanism involves the inhibition of protein synthesis, disruption of cellular function, and eventual cell death. Additionally, aminoglycosides can induce renal vasoconstriction, further exacerbating renal impairment.

**Why Each Wrong Option is Incorrect**
**Option B:** Vancomycin is associated with nephrotoxicity, but it is less common compared to aminoglycosides and is often related to the administration of high doses or prolonged infusion times.
**Option C:** Ceftriaxone, a third-generation cephalosporin, is generally considered safe in terms of nephrotoxicity and is not typically associated with renal damage.
**Option D:** Metronidazole, an antibiotic used to treat various infections, is not commonly linked with nephrotoxicity and is generally well-tolerated in terms of renal function.

**Clinical Pearl / High-Yield Fact**
When administering aminoglycosides, it is essential to monitor renal function closely, particularly in patients with pre-existing renal impairment or those receiving concomitant nephrotoxic agents.

**Correct Answer:** A. Gentamicin.