Volume of distribution calculation — FRCA Primary MCQ

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EasyPharmacologyVolume of distribution calculationFRCA Primary

A 90 kg patient receives a drug as a 900 mg intravenous bolus. The apparent volume of distribution is 0.2 L kg-1 and a one-compartment model is assumed. What is the initial plasma concentration?

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Correct answer: B — 25 mg L-1

Initial concentration equals dose divided by volume of distribution. The Vd is 0.2 L kg-1 x 90 kg = 18 L, and 900 mg/18 L = 50 mg L-1. The tempting error is to divide by body weight rather than Vd. This calculation is a core pharmacokinetic step before considering elimination.

Reference: RCoA Primary FRCA Syllabus 2026; Peck, Hill & Williams Pharmacology for Anaesthesia