Cyanide toxicity causes cellular asphyxia by binding to which component of the mitochondrial electron transport chain?
- A Complex I (NADH dehydrogenase) — blocking NADH oxidation
- B Complex IV (cytochrome c oxidase) — blocking electron transfer to oxygen ✓
- C Complex III (cytochrome bc1) — blocking ubiquinone transfer
- D ATP synthase (Complex V) — blocking phosphorylation
Correct answer: B. Complex IV (cytochrome c oxidase) — blocking electron transfer to oxygen
Explanation
Cyanide (CN⁻) binds the ferric (Fe³⁺) form of cytochrome a3 in Complex IV (cytochrome c oxidase), preventing electron transfer to molecular oxygen. This arrests aerobic respiration, causing cytotoxic (histotoxic) hypoxia where oxygen is present in tissues but cannot be utilised — explaining the characteristic cherry-red appearance of blood and tissues (venous blood is oxygen-saturated). The antidote (hydroxocobalamin or sodium nitrite/thiosulphate) works upstream at cyanide chelation.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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