A patient receiving four-drug antitubercular therapy complains of difficulty distinguishing red from green colours after 3 months. Visual acuity testing reveals reduced vision. Which drug is responsible, and what is the appropriate action?

• A Isoniazid, continue therapy with pyridoxine supplementation
• B Rifampicin, reduce the dose by half
• C Streptomycin, switch to kanamycin
• D Ethambutol, stop the drug immediately ✓

Explanation

Ethambutol causes dose-dependent retrobulbar neuritis presenting with loss of red-green colour discrimination, reduced visual acuity and central scotoma. The risk rises with doses above 15 mg/kg/day and with renal impairment. Because early changes may be reversible, the drug must be stopped promptly at the first visual symptom. Isoniazid causes peripheral neuropathy preventable with pyridoxine, while streptomycin causes ototoxicity and nephrotoxicity.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.