Which term describes a compound that binds to the same site as the agonist, inhibiting binding and effect (steric mechanism)?

• A. Noncompetitive antagonist
• B. Irreversible antagonist
• C. Physiological antagonist
• D. Competitive antagonist

Answer: (D)

This item tests competitive antagonism. A compound that binds to the same receptor site as the agonist blocks access to the binding pocket by steric hindrance, preventing activation of the receptor. Because this blockade is reversible and the antagonist and agonist compete for the same site, increasing the concentration of the agonist can outcompete the antagonist. The overall maximal effect remains possible (Emax unchanged), but you need more agonist to achieve a given response, so the dose–response curve shifts to the right, reflecting decreased apparent potency. In contrast, noncompetitive antagonists bind to a different site or inactivate the receptor, lowering Emax and not being surmountable by more agonist; irreversible antagonists bind permanently, and physiological antagonists work through opposing pathways rather than direct competition at the receptor. So the correct term is competitive antagonist.