A 24-year-old man is brought to the emergency department with cyanosis and dyspnoea after ingestion of an oxidising drug. His lips and nail beds appear grey-blue, his venous blood looks chocolate brown, and his pulse oximeter shows a fixed saturation of about 85% despite supplemental oxygen. The underlying defect is:
- A Conversion of ferrous iron to ferric iron, rendering haemoglobin unable to bind oxygen ✓
- B Formation of a complex with haemoglobin that mimics oxyhaemoglobin in light absorption
- C Inhibition of cytochrome oxidase preventing tissue oxygen utilisation
- D Denaturation of globin chains causing premature destruction of red cells
Correct answer: A. Conversion of ferrous iron to ferric iron, rendering haemoglobin unable to bind oxygen
Explanation
Methaemoglobinaemia results from oxidation of haem iron from Fe2+ to Fe3+. Ferric iron cannot bind oxygen and also increases affinity of adjacent normal subunits, shifting the curve left and impairing unloading. This produces functional anaemia with chocolate-brown blood and a saturation stuck near 85%. Cyanide poisons cytochrome oxidase (option C) but does not discolour blood or fix oximetry. Treatment is methylene blue, which uses the NADH reductase pathway.
Reference: Ganong's Review of Medical Physiology, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.