Which drug is not responsible for red/orange discoloration of body fluids in TB therapy?

• A. Isoniazid
• B. Rifampin
• C. Pyrazinamide
• D. Ethambutol

Answer: (D)

Discoloration of body fluids is a drug-specific effect, with rifampin being the classic cause of red-orange staining in urine, sweat, tears, and other secretions. This happens because rifampin itself is a red-orange dye that is excreted in these fluids. Ethambutol does not produce this effect; its main toxicity is optic neuritis with visual changes, especially impaired color discrimination, not fluid discoloration. Isoniazid and pyrazinamide are not known for changing the color of bodily fluids in routine therapy, though they have other notable toxicities (hepatotoxicity, neuropathy for isoniazid; hepatotoxicity and hyperuricemia for pyrazinamide). Therefore, the drug not responsible for red/orange discoloration of body fluids is ethambutol.