During a pharmacology experiment, a drug produces the following effects: bradycardia, increased GI peristalsis, bronchoconstriction, and pupillary constriction, but does NOT cross the blood-brain barrier. These effects are blocked by atropine. What is the drug's mechanism of action?

• A Irreversible cholinesterase inhibitor that increases ACh at muscarinic and nicotinic receptors peripherally
• B Direct muscarinic receptor agonist (peripheral, quaternary ammonium compound) ✓
• C Sympathetic ganglion blocker reducing norepinephrine release and unmasking parasympathetic tone
• D Beta-1 adrenergic blocker causing bradycardia with parasympathomimetic GI and bronchial effects

Explanation

All listed effects (bradycardia, miosis, bronchoconstriction, increased peristalsis) are muscarinic receptor-mediated parasympathomimetic effects. Complete blockade by atropine confirms muscarinic mediation. The drug does not cross the BBB (excluding tertiary amine muscarinic agonists like pilocarpine), suggesting it is a quaternary ammonium compound (e.g., bethanechol). A cholinesterase inhibitor (option A) would also activate nicotinic receptors (causing muscle fasciculations, tachycardia at ganglia) and would not be entirely blocked by atropine at nicotinic sites. A ganglionic blocker (option C) would reduce all autonomic tone, not selectively increase parasympathetic effects. A beta-1 blocker (option D) would not cause miosis or bronchoconstriction.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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