A 24-year-old man on intensive phase anti-tuberculosis therapy complains that he can no longer distinguish red from green colours. Which drug is responsible?
- A Isoniazid
- B Pyrazinamide
- C Ethambutol ✓
- D Streptomycin
Correct answer: C. Ethambutol
Explanation
Ethambutol causes a dose-dependent retrobulbar neuritis whose earliest manifestation is loss of red-green colour discrimination, followed by reduced visual acuity and central scotoma. The defect is usually reversible if the drug is stopped promptly, so patients should have monthly visual acuity and colour vision checks. Isoniazid causes peripheral neuropathy and hepatitis, pyrazinamide causes hyperuricaemia and hepatitis, and streptomycin causes ototoxicity and nephrotoxicity, none of which affect colour vision.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.