Differentiate a full agonist from a partial agonist in terms of maximal effect and receptor activation.

• A. Full agonist produces submaximal effect; partial agonist yields maximal effect.
• B. Full agonist produces the maximal possible effect; partial agonist yields a submaximal effect even when receptor occupancy is high.
• C. Both achieve maximal response if dose is high enough.
• D. Partial agonist has higher efficacy than a full agonist.

Answer: (B)

Efficacy determines how fully a receptor is activated and the resulting maximal response. A full agonist has high intrinsic activity and can drive receptors into the fully active state. When it achieves full receptor occupancy, the system can reach the maximal possible effect. A partial agonist, however, has lower intrinsic activity and only partially activates the receptor. Even when all receptors are occupied, the receptor activation is incomplete, so the overall response is submaximal. In some systems with receptor reserve, a partial agonist can produce a near-maximal effect at low occupancy, but its ceiling is still lower than that of a full agonist due to its lower intrinsic activity.