Answer: The correct answer is C. 62.5%
Explanation:
Since the two drugs have equal affinity for the same receptor, when both are present at the same time in maximally effective concentrations, one half of the receptors will be occupied by the first drug and one half by the second drug. Since the intrinsic activity of the first drug is 1.0, each receptor occupied by this drug will be activated 100% and therefore the total response of one half of the receptors will be 50% of the maximal response. By the same token, since the Intrinsic activity of the second drug is 0.25, each receptor occupied by this drug will be activated 25% and so the total response of the other half of the receptors will be 12.5% of the maximal response. Therefore the response of all receptors will be 50% +12.5%, that is 62.5 % of the maximal response.
