A 40-year-old man on intensive phase antitubercular therapy notices he cannot distinguish red from green traffic signals. Visual acuity testing shows reduced acuity bilaterally. Which drug is responsible?
- A Isoniazid
- B Ethambutol ✓
- C Streptomycin
- D Pyrazinamide
Correct answer: B. Ethambutol
Explanation
Ethambutol causes dose-dependent retrobulbar neuritis presenting with loss of red-green colour discrimination and reduced visual acuity, typically after 1 to 2 months of therapy. The risk rises with doses above 15 mg/kg/day and in renal failure because the drug accumulates. Vision usually recovers if the drug is stopped early. Isoniazid causes peripheral neuropathy and streptomycin causes ototoxicity, neither of which affects colour vision.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.