## Mechanism of Cyanide Toxicity **Key Point:** Cyanide is a rapid-acting cellular poison that blocks aerobic respiration at the mitochondrial level, leading to metabolic acidosis—not alkalosis. ### Why Option 3 (Correct Answer) is Wrong Cyanide poisoning causes **metabolic acidosis**, not alkalosis. The mechanism is: 1. Cyanide binds irreversibly to cytochrome c oxidase (Complex IV) 2. Electron transport chain is blocked 3. Cells shift to anaerobic metabolism 4. Lactic acid accumulates → metabolic acidosis 5. Tissues become hypoxic despite adequate oxygen delivery (histotoxic hypoxia) **High-Yield:** The arterial blood gas in cyanide poisoning shows metabolic acidosis with elevated lactate and normal or high venous O₂ saturation (tissues cannot use oxygen). ### Why Options 1, 2, and 4 are Correct | Feature | Details | |---------|----------| | **Cytochrome c oxidase binding** | Cyanide binds to Fe³⁺ in heme a₃ of Complex IV, blocking electron transfer | | **Pink/cherry-red skin** | High venous O₂ saturation because tissues cannot extract oxygen (histotoxic hypoxia); oxygenated hemoglobin gives pink color | | **Sodium thiosulfate** | Sulfur donor; rhodanese (mitochondrial enzyme) converts CN⁻ + S₂O₃²⁻ → SCN⁻ + SO₃²⁻ | **Clinical Pearl:** The combination of metabolic acidosis + normal/high venous oxygen saturation is pathognomonic for cyanide poisoning and distinguishes it from cardiogenic or septic shock.
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