Why is the apparent volume of distribution often larger than total body water for many drugs?

• A. Increased plasma volume.
• B. Vd is always equal to body water.
• C. Tissue binding and sequestration into tissues raises apparent volume of distribution.
• D. High renal clearance reduces Vd.

Answer: (C)

The key idea is that apparent volume of distribution reflects how widely a drug distributes beyond the plasma. If most of the drug leaves the blood and resides in tissues or binds strongly to tissue components, the plasma concentration for a given dose drops. Since Vd = Dose divided by the plasma concentration, extensive tissue distribution makes this ratio large. That tissue binding and sequestration into various tissues (often driven by lipophilicity and affinity for tissue components) pushes the apparent Vd above total body water.

Other options don’t fit as well because plasma volume expansion affects the circulating fluid compartment but doesn’t explain large tissue distribution; Vd is not fixed to body water for all drugs; and renal clearance relates to elimination, not the extent of distribution.