A competitive antagonist shifts the agonist dose-response curve in which direction without altering the maximal efficacy ()?

Answer and explanation

Correct answer: A. Parallel shift to the right (increases )

  1. Step 1: Competitive antagonists reversibly bind the active receptor site, competing directly with agonist molecules. Because higher agonist concentrations can outcompete the antagonist, the maximal system efficacy () remains achievable.
  2. Step 2: The necessity for higher agonist concentrations shifts the dose-response curve in parallel to the right, which increases the effective concentration producing maximal response () and decreases agonist potency.
Answer
Parallel shift to the right (increases ).
Why not the others: Downward shift (decreases ) is characteristic of non-competitive (allosteric or irreversible) antagonists, which reduce maximal efficacy without rightward parallel shifts; Shift to the left (decreases ) indicates an increase in agonist potency (e.g., positive allosteric modulators); Both rightward and downward shift reflects insurmountable non-competitive antagonism in the absence of spare receptors.

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