Which of the following best describes a competitive antagonist?

• A. Binds irreversibly to a separate allosteric site and blocks signaling.
• B. Binds to the receptor to produce its own effect.
• C. Enhances the affinity of the agonist for the receptor.
• D. Binds reversibly to the same site as the agonist, blocking binding and lacking intrinsic activity.

Answer: (D)

Competitive antagonism is when a molecule competes with the agonist for the same receptor binding site. It binds reversibly and has no intrinsic activity, so it blocks receptor activation only by occupying the site. Because the interaction is reversible, increasing the concentration of the agonist can outcompete the antagonist and restore the response. This type of antagonist does not change the maximal effect achievable, but it shifts the dose–response curve to the right, reflecting a higher agonist concentration is needed. The other options describe irreversible/allosteric antagonism, a direct agonist effect, or a positive modulator, none of which match competitive antagonism.