**Question:** Which of the following anti-tubercular drugs requires dose adjustment in renal failure:

A. Rifampicin
B. Isoniazid
C. Ethambutol
D. Pyrazinamide

**Core Concept:** Renal function affects the clearance of anti-tubercular drugs, which can lead to suboptimal drug levels and potentially decreased efficacy or increased toxicity.

**Why the Correct Answer is Right:**
In renal failure, the clearance of anti-tubercular drugs is reduced due to decreased glomerular filtration and tubular secretion. This results in higher drug concentrations in the blood, potentially causing toxicity.

**Why Each Wrong Option is Incorrect:**
A. Rifampicin: The clearance of rifampicin is reduced in severe renal impairment, but dose adjustments are generally not required unless creatinine clearance is <10 mL/min. B. Isoniazid: Isoniazid clearance is not significantly affected by renal function, and dose adjustment is usually not necessary. C. Ethambutol: Ethambutol clearance is minimally affected by renal dysfunction, and dose adjustment is generally not required. D. Pyrazinamide: The clearance of pyrazinamide is reduced in severe renal impairment, but dose adjustments are not routinely necessary unless creatinine clearance is <30 mL/min. **Clinical Pearl:** In patients with severe renal impairment (creatinine clearance <30 mL/min), dose adjustments of anti-tubercular drugs, including rifampicin, may be necessary to avoid drug toxicity. Close monitoring of drug concentrations and renal function is essential in these cases. **Correct Answer:** D. Pyrazinamide **Explanation:** Pyrazinamide is eliminated primarily through tubular secretion, which is reduced in severe renal impairment. Therefore, dose adjustment of pyrazinamide is required in patients with creatinine clearance <30 mL/min to prevent toxicity due to increased drug concentrations.