A 40-year-old man with drug-sensitive pulmonary TB is being treated with HRZE. At week 6, he notices progressive blurring of vision with difficulty distinguishing red from green colours. Which drug is most likely responsible?

• A Isoniazid — peripheral optic neuropathy due to pyridoxine deficiency
• B Ethambutol — retrobulbar optic neuritis causing central scotoma and red-green colour blindness ✓
• C Rifampicin — direct toxicity to retinal ganglion cells
• D Pyrazinamide — uric acid crystal deposits in the optic nerve

Explanation

Ethambutol causes retrobulbar optic neuritis characterised by central scotoma and selective loss of red-green colour discrimination, which is often the earliest symptom. It is dose-dependent and requires regular ophthalmological monitoring, especially at doses above 15 mg/kg/day. Isoniazid neuropathy is peripheral and sensorimotor, affecting the limbs. Pyrazinamide causes hyperuricaemia and arthralgia but not ocular toxicity. Rifampicin causes hepatotoxicity and drug interactions.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.