A 46-year-old man on the intensive phase of antitubercular therapy (isoniazid, rifampicin, pyrazinamide, ethambutol) returns after 6 weeks complaining he can no longer distinguish red from green traffic lights and has blurred vision. Which drug is responsible?
- A Isoniazid
- B Ethambutol ✓
- C Pyrazinamide
- D Rifampicin
Correct answer: B. Ethambutol
Explanation
Ethambutol causes dose-dependent retrobulbar neuritis presenting as loss of colour discrimination, typically red-green, with reduced visual acuity; risk rises with doses above 15 mg/kg and impaired renal function, so the drug should be stopped and the patient referred for ophthalmological assessment. Isoniazid causes peripheral neuropathy and hepatitis, prevented partly by pyridoxine supplementation. Rifampicin causes orange discoloration of secretions and hepatotoxicity. Pyrazinamide causes hyperuricaemia, arthralgia, and hepatotoxicity but not optic nerve damage.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
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Written and medically reviewed by the StethoPrep medical team.