Which statement best describes a partial agonist?

Which statement best describes a partial agonist?

• A. Activates a receptor fully
• B. Binds to receptor but prevents activation
• C. Binds to receptor or activates the receptor but not as much as a full agonist
• D. Does not interact with receptor

Answer: (C)

The key idea here is intrinsic efficacy—the ability of a bound ligand to activate the receptor and produce a response. A partial agonist binds to the receptor and can activate it, but the strength of that activation is less than what a full agonist would produce. So even when all receptors are occupied, the maximal response is submaximal. That makes the statement describing binding with activation but less than a full agonist’s effect the best description. In contrast, a full agonist would produce the maximum possible response, an antagonist would bind without activating, and a non-interacting ligand would have no effect.

The key idea here is intrinsic efficacy—the ability of a bound ligand to activate the receptor and produce a response. A partial agonist binds to the receptor and can activate it, but the strength of that activation is less than what a full agonist would produce. So even when all receptors are occupied, the maximal response is submaximal. That makes the statement describing binding with activation but less than a full agonist’s effect the best description. In contrast, a full agonist would produce the maximum possible response, an antagonist would bind without activating, and a non-interacting ligand would have no effect.