What makes carbon monoxide poisonous?

• A. Binds irreversibly to haemoglobin
• B. Reacts with carbon dioxide in blood
• C. Inhibits respiration by blocking ATP production
• D. Is completely eliminated by the lungs unchanged

Answer: (A)

Carbon monoxide poisons mainly by occupying the binding sites on haemoglobin in red blood cells. It binds far more strongly to haemoglobin than oxygen does, forming carboxyhaemoglobin. Because CO holds onto haemoglobin so tightly, less oxygen can be carried in the blood, and the oxygen that is bound is released less readily to tissues (a leftward shift of the oxygen–haemoglobin dissociation curve). This combination leads to tissue hypoxia, which causes the harmful effects of CO exposure.

The other ideas don’t explain the danger as directly: CO itself isn’t reacting with carbon dioxide in the blood to cause harm, it isn’t about blocking ATP production in a general way (that would describe something like cyanide), and CO isn’t removed from the body unchanged by the lungs—it's gradually expelled as it dissociates from haemoglobin.