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Fentanyl Potency — FRCA Primary MCQ

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HardPharmacology - AnalgesicsFentanyl PotencyFRCA Primary

A 68-year-old receives repeated fentanyl boluses during a six-hour laparotomy. He initially obeys commands, then becomes progressively somnolent with a respiratory rate of 5 min−1 in recovery. Temperature and glucose are normal. Which property best explains this delayed recurrence?

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Correct answer: D — Delayed release from accumulated peripheral fentanyl stores

The best answer is “Delayed release from accumulated peripheral fentanyl stores”. Fentanyl is highly lipophilic: repeated exposure fills peripheral compartments, and drug can return to plasma after an early fall caused by redistribution. Its terminal elimination is prolonged, so respiratory depression may recur after apparent awakening. Plasma-esterase hydrolysis describes remifentanil, morphine-6-glucuronide is not a fentanyl metabolite, and receptor binding is reversible.

Reference: Fentanyl 50 micrograms/mL solution for injection, UK SmPC: https://www.medicines.org.uk/emc/product/3387/smpc; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf