A drug is given as an IV bolus of 400 mg. Plasma concentration immediately after injection is 8 mg/L. Calculate the apparent volume of distribution (Vd) and select the correct option.

• A 50 L ✓
• B 25 L
• C 100 L
• D 0.5 L

Explanation

Vd = Dose / C0 = 400 mg / 8 mg/L = 50 L. A Vd of 50 L exceeds plasma volume (3–5 L) and blood volume (~5 L), suggesting moderate tissue distribution beyond the central compartment. A very high Vd (hundreds of litres) would imply extensive tissue accumulation; 50 L is consistent with distribution into extracellular fluid (14 L in a 70 kg person) plus some tissue binding.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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