A 46-year-old man on intensive phase antitubercular therapy (2HRZE) for one month reports difficulty distinguishing traffic signal colours. Which drug is responsible, and what is the appropriate action?
- A Ethambutol, stop the drug and refer for visual assessment ✓
- B Pyrazinamide, check liver function tests
- C Isoniazid, add pyridoxine
- D Rifampicin, continue with reassurance
Correct answer: A. Ethambutol, stop the drug and refer for visual assessment
Explanation
Ethambutol causes dose-dependent retrobulbar optic neuritis presenting as loss of red-green colour discrimination, reduced visual acuity and central scotomata. The drug must be stopped immediately and ophthalmology review arranged, since continuation risks irreversible blindness. Isoniazid causes peripheral neuropathy prevented by pyridoxine, pyrazinamide causes hepatotoxicity and hyperuricaemia, and rifampicin causes orange discolouration of secretions and drug interactions, not ocular toxicity.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.