A 50-year-old man on anti-TB therapy complains of difficulty distinguishing red from green colours after four months of treatment. Visual acuity testing reveals bilateral reduction. Which drug is responsible, and what is its mechanism of action?

• A Isoniazid, inhibition of mycolic acid synthesis
• B Rifampicin, inhibition of DNA-dependent RNA polymerase
• C Pyrazinamide, disruption of mycobacterial membrane transport
• D Ethambutol, inhibition of arabinosyl transferase in cell wall synthesis ✓

Explanation

Ethambutol causes dose-dependent retrobulbar optic neuritis presenting with loss of red-green colour discrimination, reduced visual acuity, and central scotoma. Risk rises with doses above 15 mg/kg/day and with renal impairment. Ethambutol acts by inhibiting arabinosyl transferase, blocking arabinogalactan synthesis in the mycobacterial cell wall. Isoniazid causes peripheral neuropathy rather than ocular toxicity, and rifampicin's classic adverse effects are hepatotoxicity and orange discolouration of body fluids.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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