Competitive Antagonism — FRCA Primary MCQ
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Correct answer: A — A rightward shift with no change in the maximum response
Explanation lettering: C = shown as A · D = shown as B · E = shown as C · B = shown as D · A = shown as E
A reversible competitive antagonist competes with the agonist for mutually exclusive receptor binding. At equilibrium, its effect is surmountable by increasing the agonist concentration. It therefore increases the agonist EC50 and reduces apparent potency, producing a parallel rightward shift, while efficacy and the maximum response remain unchanged. Thus C is correct. A reduced maximum without displacement (A) is not the characteristic competitive pattern. A leftward shift (B) would indicate increased apparent potency. Altering only the slope (D) is not the defining effect. A rightward shift accompanied by a reduced maximum (E) suggests insurmountable antagonism, such as non-competitive or irreversible antagonism under appropriate experimental conditions.
Reference: Wyllie DJA, Chen PE. Taking the time to study competitive antagonism. British Journal of Pharmacology. 2007;150(5):541-551. https://pubmed.ncbi.nlm.nih.gov/17245371/