A man is found dead with a smell of almonds from his mouth. Autopsy reveals bright red blood and viscera. Cyanide poisoning is suspected. Which postmortem biochemical mechanism BEST explains the bright red appearance of venous blood in cyanide poisoning?

• A Cyanide combines with haemoglobin to form cyanmethaemoglobin, which is bright red
• B Cyanide blocks cytochrome oxidase; cells cannot extract oxygen, so venous blood remains oxyhaemoglobin-rich and bright red ✓
• C Cyanide causes massive haemolysis, releasing free oxyhaemoglobin
• D Cyanide oxidizes deoxyhaemoglobin to methaemoglobin which appears bright red

Explanation

Cyanide inhibits cytochrome c oxidase (Complex IV), blocking cellular utilisation of oxygen. Consequently, tissues cannot extract O2 from blood even though delivery is normal; venous blood thus retains oxyhemoglobin and appears bright red (arterialised venous blood). This is a histotoxic hypoxia. Cyanide does not form a large amount of cyanmethaemoglobin under typical poisoning conditions; methaemoglobin is brownish, not bright red; haemolysis is not the mechanism.

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.