Which toxin interferes with the use of oxygen at the cellular level?

• A. Opiates
• B. Acids
• C. Carbon monoxide poisoning
• D. Cyanide

Answer: (D)

Cyanide blocks the cell’s ability to use oxygen by inhibiting a key mitochondrial enzyme in the electron transport chain. It binds to cytochrome c oxidase (complex IV), stopping electrons from flowing to oxygen. Without that final step, oxidative phosphorylation can’t occur, so ATP production collapses even though oxygen is present in the blood. Cells then rely on less efficient anaerobic metabolism, producing lactic acid and leading to tissue hypoxia and rapid organ failure.

Other options don’t fit this specific mechanism. Opiates can cause respiratory depression, reducing oxygen delivery to tissues but not blocking cellular use. Acids can cause metabolic disturbances, but not by blocking the cellular machinery that uses oxygen. Carbon monoxide poisoning reduces oxygen delivery by binding hemoglobin, so the problem is lack of oxygen reaching tissues, not the cells’ ability to use it.