A 45-year-old woman started on four-drug antitubercular therapy reports difficulty matching colours of her clothes three weeks later. Visual acuity testing shows reduced discrimination of red and green. Which drug is responsible?

• A Isoniazid
• B Rifampicin
• C Pyrazinamide
• D Ethambutol ✓

Explanation

Ethambutol causes a dose-dependent retrobulbar optic neuropathy whose earliest manifestation is loss of red-green colour discrimination, followed by reduced visual acuity and scotomas. The risk rises above 15 mg/kg/day, so baseline visual assessment and monthly symptom enquiry are advised. Streptomycin causes ototoxicity and nephrotoxicity, isoniazid causes peripheral neuropathy and hepatitis, and pyrazinamide mainly causes hepatotoxicity and hyperuricaemia.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.