A 34-year-old man started on anti-TB therapy returns after 4 weeks complaining that he cannot distinguish red from green traffic signals and has blurred vision. Which drug is responsible, and what monitoring would have detected this earliest?
- A Isoniazid, monthly liver function tests
- B Streptomycin, audiometry at baseline and monthly
- C Ethambutol, baseline and monthly color vision and visual acuity testing ✓
- D Pyrazinamide, serum uric acid levels
Correct answer: C. Ethambutol, baseline and monthly color vision and visual acuity testing
Explanation
Ethambutol causes a dose-dependent retrobulbar optic neuritis. The earliest detectable change is loss of red-green color discrimination, followed by reduced visual acuity and scotomas. Baseline and monthly color vision and acuity testing allow detection before permanent damage. The deficit is usually reversible if the drug is stopped early. Isoniazid causes ocular toxicity rarely and presents differently, while streptomycin damages hearing and balance, not color vision.
Reference: Katzung Basic and Clinical Pharmacology, 16th ed.
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Written and medically reviewed by the StethoPrep medical team.