An antagonist medication:

• A. Binds to a receptor and blocks other medications or chemicals from attaching there.
• B. Activates receptors to cause a response.
• C. Increases receptor sensitivity to all ligands.
• D. Is always unrelated to receptors.

Answer: (A)

Antagonist medications work by occupying receptor binding sites and preventing other substances from attaching, so no activation occurs. This blocks the effect of agonists or endogenous ligands without producing a response themselves. For example, an antagonist at opioid receptors prevents opioids from producing their effects, and a beta-blocker antagonist prevents stimulation of beta receptors from causing increased heart rate or blood pressure. The other ideas describe activation of receptors (an agonist), or changing receptor sensitivity in ways not characteristic of antagonists, or functioning unrelated to receptors, which isn’t accurate for antagonists.