A client frequently takes diphenhydramine (Benadryl) to alleviate his upper respiratory symptoms associated with allergies. Which of the following mechanisms of action applies to diphenhydramine (Benadryl)?

The correct answer is D because diphenhydramine (Benadryl) is a first-generation antihistamine that works by blocking histamine receptors, specifically the H1 receptors. By binding to these receptors, diphenhydramine inhibits the effects of histamine, thus reducing symptoms such as sneezing, itching, and nasal congestion. This mechanism of action is effective in alleviating upper respiratory symptoms associated with allergies.
Choice A is incorrect because diphenhydramine does not stimulate salivation. Choice B is incorrect as diphenhydramine blocks H1 receptors, not H4 receptors. Choice C is incorrect as diphenhydramine does not stimulate the effects of histamine.