Cyanide poisoning causes histotoxic hypoxia.

• A. Hypoxic hypoxia
• B. Hypemic hypoxia
• C. Stagnant hypoxia
• D. Histotoxic hypoxia

Answer: (D)

Cyanide poisoning blocks the cell’s ability to use oxygen by inhibiting cytochrome oxidase in mitochondria. That shuts down oxidative phosphorylation, so even though oxygen is delivered to tissues and arterial oxygen levels may be normal, the cells cannot extract or utilize that oxygen. This is histotoxic hypoxia: the problem is not the amount of oxygen in the blood, but the tissue’s inability to metabolize it due to a toxin. You’ll often see metabolic signs of anaerobic metabolism, like lactic acidosis, because the cells switch to less efficient energy production. Context helps: other hypoxia types involve less oxygen delivery (hypoxic), reduced oxygen-carrying capacity (hypemic), or impaired perfusion (stagnant), but in histotoxic hypoxia, the oxygen isn’t used despite adequate delivery.