Direct Sympathomimetics — FRCA Primary MCQ
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Correct answer: B — Phenylephrine
Phenylephrine is correct. It acts almost exclusively by directly stimulating postsynaptic alpha-1-adrenoceptors, producing arterial and venous vasoconstriction, increased systemic vascular resistance and reflex bradycardia. Ephedrine is mixed-acting: it directly stimulates alpha and beta receptors and indirectly releases noradrenaline from sympathetic nerve terminals. Metaraminol is also mixed-acting; although its direct alpha-mediated vasoconstrictor effect is prominent, it displaces neuronal noradrenaline and can act as a false transmitter. Amphetamine and tyramine are principally indirect sympathomimetics that increase the availability of endogenous catecholamines rather than primarily activating postsynaptic adrenoceptors. The original stem was insufficiently precise because an agent with mixed activity still has a direct-acting component.
Reference: Electronic Medicines Compendium, Phenylephrine 10 mg/ml concentrate for solution for injection/infusion, SmPC section 5.1 Pharmacodynamic properties, 2025. https://www.medicines.org.uk/emc/product/10818/smpc