A patient develops cyanosis unresponsive to oxygen therapy after starting dapsone. The blood sample appears chocolate-brown and does not turn red on exposure to air. The primary enzymatic pathway that reduces methemoglobin (Fe3+) back to functional hemoglobin (Fe2+) in erythrocytes uses which electron donor?
- A NADPH
- B NADH ✓
- C Reduced glutathione
- D Ascorbate
Correct answer: B. NADH
Explanation
NADH-cytochrome b5 reductase is the primary methemoglobin reductase, accounting for about two-thirds of reduction capacity. The minor NADPH-methemoglobin reductase pathway requires an artificial electron carrier like methylene blue to function. Dapsone oxidizes hemoglobin to methemoglobin, overwhelming the NADH-dependent pathway.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.