A 35-year-old man is found dead after consuming an unknown substance. Autopsy reveals cherry-red discolouration of blood and tissues, petechiae in the brain, and no specific odour. The COHb (carboxyhemoglobin) saturation is 72%. Which finding on post-mortem chemistry MOST distinguishes carbon monoxide poisoning from cyanide poisoning, both of which cause cellular asphyxia?
- A Elevated blood lactate in CO poisoning only
- B Cherry-red colour is unique to CO and never seen in cyanide poisoning
- C Elevated COHb with normal cytochrome oxidase activity in CO versus blocked cytochrome oxidase with normal COHb in cyanide ✓
- D Cyanide poisoning shows elevated COHb because it stimulates haemoglobin oxidation
Correct answer: C. Elevated COHb with normal cytochrome oxidase activity in CO versus blocked cytochrome oxidase with normal COHb in cyanide
Explanation
CO binds haemoglobin to form COHb (measured by spectrophotometry), causing impaired oxygen transport; cytochrome oxidase function is normal at usual CO levels. Cyanide directly inhibits cytochrome aa3 (complex IV), blocking mitochondrial electron transport; haemoglobin remains oxygenated (bright red venous blood), and COHb is not elevated. Both can produce cherry-red appearance because cyanide leaves blood fully oxygenated, not due to COHb formation. Lactate rises in both but is not the distinguishing marker.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.