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Chirality and Enantiomers — FRCA Primary MCQ

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HardPharmacology - General PrinciplesChirality and EnantiomersFRCA Primary

A protocol substitutes S-ketamine for racemic ketamine while aiming for a comparable dissociative effect. Which pharmacological fact is most relevant when selecting the new dose?

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Correct answer: E — S-ketamine has substantially greater analgesic-anaesthetic potency than R-ketamine

The best answer is “S-ketamine has substantially greater analgesic-anaesthetic potency than R-ketamine”. Chiral molecules can interact differently with stereoselective biological targets despite sharing a molecular formula. The UK SmPC describes an approximate 1:3 potency relation between R- and S-ketamine, with S-ketamine more potent, principally through NMDA-receptor antagonism. A direct milligram-for-milligram substitution can therefore produce unintended depth unless dose and clinical response are reconsidered.

Reference: Esketamine 25 mg/mL solution for injection/infusion, UK SmPC: https://www.medicines.org.uk/emc/product/13195/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