What describes an antagonist?

• A. A molecule that binds a receptor and produces a physiological response
• B. A molecule that binds to a receptor and decreases the effect of an agonist; does not produce any response by itself
• C. A molecule that enhances endogenous ligand release
• D. A molecule that blocks receptor function by changing its shape

Answer: (B)

An antagonist is a molecule that binds to a receptor but does not activate it, so it prevents or diminishes the response that an agonist would produce. The best description here states that it binds to a receptor and decreases the effect of an agonist, without producing any response on its own. This captures the essential idea: no intrinsic activity, just blocking or dampening receptor signaling.

The other descriptions refer to different concepts: an agonist produces a physiological response; a molecule that increases endogenous ligand release would enhance signaling rather than block it; and a molecule that blocks receptor function by changing its shape can describe a blocking mechanism, but it doesn’t as directly convey the core idea of lacking intrinsic activity and reducing an agonist’s effect.