An irreversible antagonist is defined as a drug that binds to the same receptor site as the agonist in an irreversible fashion.

• A. Binds reversibly to the receptor at the same site as the agonist
• B. Binds irreversibly to a different receptor site than the agonist
• C. Binds to the same site as the agonist in an irreversible fashion
• D. Acts as a chemical antagonist by binding to another drug

Answer: (C)

Irreversible antagonism is about blocking receptor activity permanently by occupying the same site the agonist would use. When an irreversible antagonist binds to the agonist binding site, it forms a very strong, essentially permanent bond to that receptor. That means those receptors are taken out of commission for as long as the cell exists or until new receptors are made. As a result, even a high concentration of agonist can’t fully overcome the blockade in the short term, and the maximum response is reduced because there are fewer functional receptors available.

This statement fits that concept exactly: it describes binding to the same site as the agonist and doing so irreversibly.

If the antagonist bound irreversibly to a different receptor site, the effect would be noncompetitive but not by occupying the same site as the agonist, which changes the interpretation. If it bound reversibly to the same site, it would be a competitive, not irreversible, antagonist. And binding to a different drug or acting through chemical antagonism isn’t about receptor-site blocking at all, so those scenarios don’t match the defined mechanism.