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First-Order Elimination — FRCA Primary MCQ

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EasyPharmacology - General PrinciplesFirst-Order EliminationFRCA Primary

A patient is given an IV bolus of 500 mg of a drug. The initial plasma concentration is 50 mg/L. After one half-life the concentration is 25 mg/L. After two half-lives the concentration will be:

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Correct answer: B — 12.5 mg/L

In first-order kinetics a constant fraction (50%) is eliminated per half-life. Starting at 50 mg/L: after 1 half-life = 25 mg/L / after 2 half-lives = 12.5 mg/L / after 3 = 6.25 mg/L / after 4 = 3.125 mg/L / after 5 = 1.5625 mg/L.

Reference: Peck Hill Williams 2014 Pharmacology for Anaesthesia and Intensive Care