Which histamine-blocking agent lacks activity at muscarinic receptors______________?
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Answer: C. Terfenadine
Explanation
Terfenadine is a selective peripheral histamine H1 receptor antagonist that does not cross the blood-brain barrier. Because of its chemical structure it fails to bind muscarinic acetylcholine receptors in autonomic tissues. This receptor selectivity explains its lack of typical anticholinergic side effects like dry mouth. First-generation antihistamines readily cross the blood-brain barrier and block various unintended receptors. Muscarinic receptors mediate parasympathetic effects controlling secretions and smooth muscle tone. Second-generation antihistamines were specifically developed to minimize central nervous system and anticholinergic toxicity.
Allergy sufferers benefit from modern antihistamines that treat symptoms without causing profound drowsiness. Clinicians prefer non-sedating agents for daytime use in patients operating machinery or vehicles. Understanding receptor profiles helps pharmacists predict potential side effect profiles of medications. Diphenhydramine is a first-generation antihistamine well known for causing significant anticholinergic side effects. Promethazine exhibits strong anticholinergic properties alongside its prominent antihistaminic activity. Pyrilamine is an older ethylenediamine antihistamine that frequently produces sedative and anticholinergic reactions.
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