Which term best describes a compound that can activate a receptor but is unable to elicit a maximal response?

• A. Full agonist
• B. Inverse agonist
• C. Partial agonist
• D. Biased agonist

Answer: (C)

Intrinsic efficacy determines how strongly a ligand can push a receptor to produce a response after binding. A full agonist has high efficacy and can drive the system to its maximum response when receptors are occupied. A compound that can activate the receptor but cannot reach the maximal response has lower efficacy; even with full receptor occupancy, the signal plateaus below the maximum. That is why this term is described as a partial agonist. It still generates a measurable response, just not a full one. By contrast, an inverse agonist would reduce baseline activity, a biased agonist preferentially activates one signaling pathway, and a full agonist would reach the full maximal effect.