A 28-year-old man receiving daily HRZE therapy for pulmonary tuberculosis reports, at his 8-week review, that he cannot distinguish red from green traffic lights. Visual acuity is mildly reduced and fundus examination is unremarkable. Which drug is responsible, and what is the lesion?

• A Isoniazid, causing central retinal degeneration
• B Streptomycin, causing cochlear toxicity
• C Ethambutol, causing retrobulbar (optic) neuritis ✓
• D Pyrazinamide, causing papilloedema

Explanation

Ethambutol produces a dose-related optic (retrobulbar) neuritis whose earliest manifestation is loss of red-green colour discrimination, followed by reduced visual acuity and loss of central scotoma; recovery usually follows prompt withdrawal. Baseline and monthly visual assessment is advised at doses above 15 mg/kg/day. The tempting distractor is isoniazid, but its toxicity is a distal symmetrical sensory peripheral neuropathy from pyridoxine deficiency, never isolated colour blindness.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.