Which statement describes the relationship among an agonist, a partial agonist, and an antagonist?

• A. An agonist fully activates receptor signaling; a partial agonist activates but yields a submaximal response; an antagonist blocks receptor signaling.
• B. An antagonist fully activates receptor signaling; a partial agonist blocks signaling; an agonist has no effect.
• C. An agonist binds with no effect.
• D. An antagonist amplifies all signaling; a partial agonist reduces signaling below baseline.

Answer: (D)

At the receptor level, three roles describe how drugs influence signaling. An agonist binds and produces the full signaling response when it occupies receptors. A partial agonist also binds and activates receptors, but the signal it generates is submaximal—even when all receptors are occupied. An antagonist binds without producing signaling of its own and prevents other molecules from activating the receptor, effectively dampening or blocking the response.

So the best description is that the agonist yields a full response, the partial agonist yields a submaximal response, and the antagonist blocks signaling. The idea that an antagonist amplifies signaling goes against its blocking role, and saying an agonist has no effect contradicts its activating action. Similarly, a partial agonist reducing signaling below baseline isn’t the standard way we describe its activity in this context.