A middle aged man with chronic renal failure is diagnosed to have sputum-positive pulmonary tuberculosis. His creatinine clearance is 25 ml/min. All of the following drugs need modification in doses except:
**Core Concept**
The question tests the understanding of drug dosing in patients with renal impairment, specifically the need for dose modification in patients with chronic renal failure. The creatinine clearance of 25 ml/min indicates moderate renal impairment, and the patient's pulmonary tuberculosis diagnosis necessitates the use of antitubercular drugs.
**Why the Correct Answer is Right**
In patients with renal impairment, the clearance of drugs that are primarily excreted by the kidneys is reduced. This can lead to increased drug concentrations and potentially toxic effects. For example, rifampicin is primarily metabolized by the liver but is excreted into the bile, and a small fraction is excreted in the urine. Its dose does not need to be modified in patients with renal impairment.
**Why Each Wrong Option is Incorrect**
* **Option A:** Some antitubercular drugs, such as isoniazid and ethambutol, are primarily excreted by the kidneys and require dose modification in patients with renal impairment.
* **Option B:** Pyrazinamide is primarily excreted by the kidneys, and its dose needs to be modified in patients with renal impairment to prevent accumulation and potential hepatotoxicity.
* **Option C:** Ethambutol is primarily excreted by the kidneys, and its dose needs to be modified in patients with renal impairment to prevent accumulation and potential optic neuritis.
**Clinical Pearl / High-Yield Fact**
In patients with renal impairment, it is essential to assess the renal function before initiating therapy with drugs that are primarily excreted by the kidneys, and to adjust the doses accordingly to prevent toxicity.
**Correct Answer:** .