A patient rescued from a house fire has confusion, hypotension, and bright cherry red discoloration of the skin despite severe hypoxia. Pulse oximetry reads falsely reassuring values. Smoke inhalation exposure to hydrogen cyanide causes cellular hypoxia chiefly by inhibiting which complex?

• A Complex IV (cytochrome c oxidase) ✓
• B Complex II (succinate dehydrogenase)
• C Complex III (cytochrome bc1)
• D Complex I (NADH dehydrogenase)

Explanation

Cyanide binds the ferric iron (Fe3+) of cytochrome aa3 in Complex IV, halting the terminal transfer of electrons to oxygen. Oxidative phosphorylation ceases, tissues switch to anaerobic glycolysis causing lactic acidosis, and venous blood remains oxygenated, producing the cherry red colour. Barbiturates and rotenone act at Complex I, antimycin D at Complex III, and malonate competitively inhibits Complex II.

Reference: Lippincott Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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