Which toxin interferes with cellular use of oxygen, causing severe hypoxia?

• A. Hydrogen sulfide
• B. Methemoglobinemia
• C. Carbon monoxide poisoning
• D. Cyanide

Answer: (D)

The key idea is histotoxic hypoxia: oxygen reaches the blood and tissues, but the cells cannot use it to make energy. Cyanide does this by binding to cytochrome oxidase in the mitochondrial electron transport chain, effectively halting oxidative phosphorylation. With the final step of cellular respiration blocked, ATP production drops dramatically, cells switch to anaerobic metabolism, lactic acid builds up, and tissues become severely hypoxic despite normal oxygen levels in the blood. This direct blockade of the cell’s ability to use oxygen is why cyanide poisoning leads to rapid, profound hypoxia.

Other toxins affect oxygen delivery rather than cellular use. Methemoglobinemia reduces how much oxygen the blood can carry; carbon monoxide lowers oxygen delivery and impairs release; hydrogen sulfide can also impair mitochondrial respiration, but cyanide is the classic agent that stops cells from utilizing oxygen at the mitochondrial level.