A 52-year-old man on intensive phase anti-TB therapy complains of difficulty distinguishing red from green colours while reading traffic signals. Which drug is responsible, and what is the underlying target?

• A Rifampicin, inhibition of DNA-dependent RNA polymerase
• B Streptomycin, binding to the 30S ribosomal subunit
• C Pyrazinamide, disruption of mycolic acid synthesis
• D Ethambutol, inhibition of mycobacterial arabinosyltransferase ✓

Explanation

Ethambutol inhibits the embB-encoded arabinosyltransferase, blocking arabinogalactan synthesis in the cell wall. Its most important dose-dependent toxicity is retrobulbar (optic) neuritis, presenting with loss of red-green colour discrimination and reduced visual acuity. Baseline and monthly visual assessment is advised. Rifampicin causes harmless orange body fluid discolouration, which is the classic distractor here, but it does not affect vision.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.