A patient receiving anti-tuberculosis therapy complains that traffic lights look washed out. Visual acuity testing shows reduced colour discrimination for red and green. Which drug is responsible, and what is monitored to detect early toxicity?

• A Rifampicin; serum transaminases
• B Streptomycin; audiometry
• C Isoniazid; serum pyridoxine levels
• D Ethambutol; visual acuity and red-green colour perception ✓

Explanation

Ethambutol causes dose-dependent retrobulbar (optic) neuritis, presenting with loss of red-green colour vision and decreased visual acuity, sometimes with a central scotoma. Baseline and monthly Snellen and Ishihara testing detects it early, and the deficit is usually reversible if the drug is stopped promptly. Rifampicin causes hepatitis and orange body fluids, while streptomycin affects hearing, not colour vision.

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

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