Cyanide poisoning differs from carbon monoxide poisoning in which of the following features?

• A Cyanide causes bright cherry-red skin discolouration; CO causes cyanosis
• B Cyanide produces high venous oxygen saturation due to impaired tissue oxygen utilization; CO produces low venous oxygen saturation ✓
• C Cyanide inhibits cytochrome a3 (complex IV); CO primarily forms carboxyhaemoglobin and also inhibits complex IV
• D Both cyanide and CO can be treated with hydroxocobalamin as specific antidotes

Explanation

Cyanide blocks cellular respiration by inhibiting cytochrome c oxidase (cytochrome a3), preventing tissues from utilising oxygen — as a result, venous blood is paradoxically bright red and oxygen-rich (venous PO2 is high, venous oxygen saturation is high) because oxygen cannot be extracted by cells. CO on the other hand causes tissue hypoxia primarily via haemoglobin binding and also inhibits complex IV, but venous oxygen saturation is low because CO shifts the dissociation curve. Hydroxocobalamin is indeed the antidote for cyanide but NOT for CO. Both produce cherry-red skin (HbCO in CO, and oxyhaemoglobin in venous blood in cyanide).

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.