An antagonist binds irreversibly to the active site of a receptor, permanently removing a fraction of the receptor population. Increasing the agonist concentration tenfold does not restore the original maximal response. On the agonist concentration-response curve this antagonist produces:

• A Parallel rightward shift with unchanged Emax
• B Rightward shift with increased slope
• C Leftward shift with increased Emax
• D Downward shift with reduced Emax ✓

Explanation

By destroying part of the receptor pool, a noncompetitive or irreversible antagonist reduces the maximal achievable effect (Emax) regardless of how much agonist is added, since no amount of agonist can recruit inactivated receptors. A parallel rightward shift with preserved Emax is the signature of reversible competitive antagonism, where raising agonist concentration can surmount the block.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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