Which poisoning prevents cells from using oxygen?

• A. Carbon Monoxide Poisoning
• B. Cyanide Poisoning
• C. Methemoglobinemia
• D. Opioid Overdose

Answer: (B)

The question tests understanding of cellular respiration being blocked by a toxin. Cyanide poisons the mitochondria by inhibiting cytochrome c oxidase (complex IV) in the electron transport chain. This stops electrons from being transferred to oxygen, so oxidative phosphorylation can’t occur and ATP production halts even though oxygen is present. Cells end up unable to use oxygen—tissues become histotoxically hypoxic, forcing a shift to anaerobic glycolysis and causing lactic acidosis and rapid organ dysfunction.

Other poisons affect oxygen delivery rather than cellular utilization: carbon monoxide and methemoglobinemia reduce the amount of oxygen reaching tissues or its release, while opioid overdose mainly lowers oxygen availability by suppressing breathing.