Which is a poison?

• A. Cyanide
• B. Acetaminophen
• C. DDT
• D. Ethylene glycol

Answer: (A)

This question tests how a chemical acts as a poison. Cyanide is a classic poison because it blocks a key step in cellular respiration. It binds to cytochrome c oxidase in the mitochondrial electron transport chain, stopping oxidative phosphorylation. Without that final step, cells can’t reliably use oxygen to make ATP, so energy production collapses even when oxygen is plentiful. This rapid energy failure—especially in high-demand tissues like the brain and heart—leads to quick and severe toxicity.

Acetaminophen is generally safe at normal doses, but overdose can injure the liver due to a toxic metabolite. DDT is a pesticide with toxic effects in exposure, but not the same rapid, universal shutdown of cellular respiration. Ethylene glycol becomes deadly because its metabolites cause metabolic acidosis and kidney damage. Cyanide’s mechanism makes it the most direct example of a poison among the options.