In a death suspected to be due to cyanide poisoning, the cherry-red colour of the skin and mucosae is caused by:

• A Carboxyhaemoglobin formation preventing oxygen dissociation
• B Methaemoglobin formation causing a shift in the oxygen dissociation curve
• C Excess oxyhaemoglobin in venous blood because tissues cannot utilise oxygen due to cytochrome c oxidase inhibition ✓
• D Haemolysis releasing free haemoglobin into the circulation

Explanation

Cyanide inhibits mitochondrial cytochrome c oxidase (complex IV), preventing oxidative phosphorylation. Since cells cannot utilise oxygen, venous blood retains a high proportion of oxyhaemoglobin (HbO2) rather than becoming deoxygenated haemoglobin. This high venous oxyhaemoglobin level produces cherry-red or bright red discolouration of the skin, mucosae, and postmortem lividity. Carbon monoxide also causes cherry-red appearance by forming carboxyhaemoglobin, but through a different mechanism. Methaemoglobin is chocolate-brown; haemolysis is not relevant here.

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.