A 65-year-old man with benign prostatic hyperplasia and chronic open-angle glaucoma receives an over-the-counter cold medication containing pseudoephedrine and diphenhydramine. He develops acute urinary retention and blurred vision. What receptor antagonism caused this urinary retention?

Answer and explanation

Correct answer: B. Muscarinic acetylcholine receptor () blockade

  1. Step 1: Identify the pharmacologic culprit: diphenhydramine is a first-generation antihistamine with strong off-target anticholinergic (antimuscarinic) properties.
  2. Step 2: Normal micturition requires parasympathetic stimulation of muscarinic receptors on the detrusor muscle, causing contraction and bladder emptying. Antagonism of receptors prevents detrusor contraction, precipitating acute urinary retention in patients with pre-existing bladder outlet obstruction (BPH).
Answer
Muscarinic acetylcholine receptor () blockade.
Why not the others: -adrenergic receptor blockade would relax the internal urethral sphincter and facilitate urination (the mechanism of tamsulosin), whereas pseudoephedrine stimulates to increase tone; Histamine receptor activation would trigger allergic signaling rather than retention; Nicotinic receptor blockade occurs at autonomic ganglia or neuromuscular junctions, producing paralysis or autonomic collapse rather than isolated urinary retention.

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