A 52-year-old man on intensive phase antitubercular therapy complains he cannot distinguish red from green thread while weaving. Visual acuity is normal. The most likely offending drug and its dose-related target are:

• A Isoniazid, hepatic acetylation pathway
• B Rifampicin, retinal pigment epithelium
• C Streptomycin, vestibular apparatus
• D Ethambutol, optic nerve fibres ✓

Explanation

Ethambutol causes dose-dependent retrobulbar neuritis; impairment of red-green colour discrimination is the earliest sign and may precede loss of visual acuity, so patients are screened monthly with colour vision testing. Isoniazid causes peripheral neuropathy and hepatitis, streptomycin causes ototoxicity and nephrotoxicity, and rifampicin causes hepatotoxicity and orange discolouration of secretions, not ocular toxicity.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.