A 32-year-old woman on dapsone for leprosy presents with dyspnoea and headache. Blood drawn for gas analysis appears chocolate brown and does not brighten on exposure to air. Pulse oximetry reads 85% and does not correct with 100% oxygen. What is the definitive treatment?

• A Intravenous methylene blue ✓
• B Hyperbaric oxygen therapy
• C Exchange transfusion
• D N-acetylcysteine infusion

Explanation

Dapsone causes oxidant-induced methemoglobinaemia: ferrous iron is converted to ferric iron, which cannot bind oxygen and gives blood its chocolate brown colour. Methylene blue provides an artificial electron acceptor via NADPH-dependent methemoglobin reductase, rapidly reducing ferric iron back to the ferrous state. Hyperbaric oxygen is the treatment for carbon monoxide poisoning, not methemoglobinaemia, and pulse oximetry characteristically reads around 85% regardless of inspired oxygen.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.