Victims rescued from a house fire present with cherry red skin, headache and confusion. Pulse oximetry reads normal despite profound tissue hypoxia. The mechanism explaining this discrepancy is:

• A Carbon monoxide oxidises haemoglobin iron to the ferric state forming methaemoglobin
• B Carbon monoxide binds haemoglobin with far greater affinity than oxygen and impairs oxygen release ✓
• C Carbon monoxide dissolves in plasma and blocks cytochrome oxidase directly
• D Carbon monoxide denatures the oxygen binding pocket of haemoglobin irreversibly

Explanation

Carbon monoxide binds haemoglobin with roughly 200 to 250 times the affinity of oxygen, forming carboxyhaemoglobin that pulse oximeters cannot distinguish from oxyhaemoglobin, so saturation readings stay falsely normal. It also shifts the oxygen dissociation curve to the left, further impairing tissue oxygen delivery. Ferric iron formation defines methaemoglobinaemia, and cyanide, not carbon monoxide, classically poisons cytochrome oxidase.

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.