A 45-year-old man on intensive phase antitubercular therapy for 5 weeks reports that he cannot distinguish the red from the green signal at traffic intersections. Visual acuity is unaffected and fundus examination is normal. Which drug should be stopped?
- A Isoniazid
- B Ethambutol ✓
- C Rifampicin
- D Pyrazinamide
Correct answer: B. Ethambutol
Explanation
Ethambutol causes dose-dependent retrobulbar neuritis, and loss of red-green colour discrimination is its earliest manifestation before visual acuity falls, so patients must be warned and colour vision tested at baseline and periodically. Isoniazid causes peripheral neuropathy and hepatitis, rifampicin causes orange discolouration of secretions and enzyme induction, and pyrazinamide causes hepatitis and hyperuricaemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.