Autonomic Pharmacology USMLE Step 1 Practice Question
A 52-year-old man with Parkinson's disease is started on benztropine 2 mg daily to manage his motor symptoms. Two weeks later, he presents to clinic complaining of difficulty initiating urination and a sensation of incomplete bladder emptying. Vital signs are stable (BP 136/84, HR 82, RR 14). Physical examination reveals a palpable suprapubic mass consistent with urinary retention. Post-void residual volume is 210 mL on bladder ultrasound. Urinalysis and renal function are normal. Which of the following best explains the urinary retention observed in this patient?
Answer choices
- ABlockade of muscarinic M1 receptors in the cerebral cortex, reducing central processing of micturition
- BBlockade of muscarinic M3 receptors on the detrusor muscle, impairing parasympathetic-mediated bladder contractionCorrect answer
- CActivation of nicotinic receptors in the parasympathetic ganglia, increasing acetylcholine sensitivity
- DInhibition of sympathetic β3-adrenergic receptors on the bladder dome, preventing urine storage
- ECompetitive antagonism of acetylcholine at the neuromuscular junction of the external urethral sphincter
- FIncreased dopaminergic signaling in the pontine micturition center, suppressing the micturition reflex
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