A family is found unconscious in a closed garage where a coal brazier had been burning overnight. Two members have bright cherry-red skin. The mechanism underlying their toxicity is best described as:
- A Formation of methaemoglobin blocking oxygen delivery
- B Binding of carbon monoxide to haemoglobin with an affinity far greater than oxygen, shifting the oxyhaemoglobin dissociation curve leftward ✓
- C Direct lipid peroxidation of neuronal membranes by the inhaled gas
- D Irreversible inhibition of cytochrome oxidase causing histotoxic hypoxia
Correct answer: B. Binding of carbon monoxide to haemoglobin with an affinity far greater than oxygen, shifting the oxyhaemoglobin dissociation curve leftward
Explanation
Carbon monoxide binds haemoglobin with roughly 200 to 250 times the affinity of oxygen, forming carboxyhaemoglobin, reduces oxygen-carrying capacity, and shifts the dissociation curve left so remaining oxygen is released less readily to tissues; the cherry-red colour comes from carboxyhaemoglobin itself. Methaemoglobinaemia is caused by nitrates and related oxidants. Cytochrome oxidase inhibition defines cyanide poisoning, the classic distractor here. Treatment is high-flow, preferably hyperbaric, oxygen.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.