An irreversible acetylating inhibitor of an enzyme-linked receptor is added to a tissue bath containing a full agonist. Increasing the agonist concentration tenfold fails to restore the original maximal response. Compared with the control curve, the agonist concentration-response curve now shows:

• A Parallel rightward shift with unchanged Emax
• B Decreased Emax with markedly increased EC50
• C Decreased Emax with little change in EC50 ✓
• D Unchanged Emax with decreased EC50

Explanation

Irreversible and noncompetitive antagonists reduce the number of functionally available receptors, lowering Emax. Because the remaining unoccupied receptors still respond normally to the agonist, the EC50 changes little, unlike competitive antagonism where a parallel right shift occurs with preserved Emax. The stem explicitly states that raising agonist concentration cannot overcome the block, which excludes simple competitive inhibition and points to loss of receptor reserve capacity.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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