A 52-year-old man on anti-TB therapy for 4 months reports difficulty distinguishing traffic light colours. Visual acuity is normal, fundoscopy shows no abnormality. Which drug is responsible?
- A Isoniazid
- B Ethambutol ✓
- C Pyrazinamide
- D Rifampicin
Correct answer: B. Ethambutol
Explanation
Ethambutol causes dose-dependent optic neuropathy, typically presenting with impaired red-green colour discrimination before loss of visual acuity, due to toxicity to the optic nerve fibres (often centrocaecal scotoma on perimetry). The effect is related to dose and duration, more common above 15 mg/kg/day. Isoniazid causes peripheral neuropathy and hepatitis, pyrazinamide causes hepatotoxicity and hyperuricaemia, and rifampicin causes orange discolouration of secretions, none of which affect colour vision.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.