A drug has a volume of distribution of 4 L in a 70 kg adult. This suggests the drug is predominantly located in which compartment?

• A Total body water
• B Extracellular fluid
• C Adipose tissue
• D Plasma ✓

Explanation

Plasma volume is approximately 3 to 4 L in an adult. A Vd of 4 L indicates the drug is largely confined to the plasma compartment, suggesting high protein binding or large molecular size preventing tissue distribution. Total body water is about 42 L, extracellular fluid about 14 L. Drugs with Vd far exceeding 42 L, like those in the question bank examples of 400 to 700 L, indicate extensive tissue binding.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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