When selecting antibiotics for patients with both liver and kidney dysfunction, it is crucial to consider the drug's metabolism and excretion pathways to avoid toxicity or subtherapeutic dosing.

- Clindamycin is primarily metabolized by the liver, and although liver impairment may require caution, dose adjustment is generally not required in renal dysfunction.
- Metronidazole undergoes extensive hepatic metabolism, and its metabolites are cleared renally. Therefore, in patients with both liver and kidney dysfunction, dose adjustment is necessary to prevent accumulation and toxicity.
- Ceftriaxone is eliminated through both biliary and renal routes, but dose adjustment is usually not needed in renal impairment alone, and caution is advised primarily in patients with severe liver disease due to biliary sludging.
- Vancomycin is almost exclusively cleared by the kidneys, requiring dose adjustment in renal dysfunction, but no significant hepatic metabolism occurs, so liver dysfunction alone does not necessitate dose modification.

Hence, among the options, Metronidazole uniquely requires dose adjustment in the presence of both liver and kidney impairment due to its dual metabolism and clearance pathways.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 42: Antimicrobial Drugs / Pharmacokinetics and Dosage Considerations