Correct Answer: C. Sequestered in body tissues
Volume of distribution (Vd) is a theoretical volume that relates the total amount of drug in the body to its plasma concentration: Vd = Total dose / Plasma concentration. A very high Vd (6 L/kg, or 420 L in a 70 kg adult) indicates that the drug is extensively distributed beyond the plasma compartment into body tissues. This occurs because the drug has a high affinity for tissue binding sites and/or is lipophilic enough to cross cell membranes readily. When Vd is very large, it means only a small fraction of the total drug remains in plasma at any given time—the majority is sequestered (trapped) in extravascular tissues. This is the hallmark of drugs like digoxin, chloroquine, and amiodarone, which accumulate in tissues and have Vd values far exceeding body water volume. The high tissue sequestration explains why plasma protein binding alone cannot account for such a large Vd; plasma proteins can only account for Vd up to 0.05 L/kg. Therefore, the correct answer is that the drug is sequestered in body tissues, not confined to plasma or bound to plasma proteins.
Why the other options are wrong
A. Highly bound to plasma proteins — This is wrong because plasma protein binding alone cannot produce a Vd of 6 L/kg. Even 100% plasma protein binding yields a maximum Vd of only 0.05 L/kg (plasma volume 3.5 L in a 70 kg adult). A Vd of 6 L/kg (420 L total) far exceeds total body water (42 L) and indicates tissue sequestration, not plasma binding. This is a common NBE trap that confuses protein binding with distribution. **B. Confined to vascular compartment** — This is the opposite of what a high Vd means. A Vd of 6 L/kg indicates extensive extravascular distribution. Drugs confined to the vascular compartment (e.g., heparin, warfarin) have Vd values close to plasma volume (0.05 L/kg). The very high Vd proves the drug has left the vascular space and accumulated in tissues, making this option factually incorrect. D. Both A and B — This is wrong because neither option A nor B is correct individually. Plasma protein binding and vascular confinement are mutually exclusive with a Vd of 6 L/kg. This option represents a common NBE strategy: combining two plausible-sounding but incorrect statements to trap students who haven't mastered the quantitative relationship between Vd and drug distribution.
High-Yield Facts
- Vd = Total dose / Plasma concentration — a mathematical relationship that reveals where drug molecules reside in the body.
- Vd > 1 L/kg indicates tissue sequestration — the drug is distributed beyond plasma into extravascular compartments (fat, muscle, organs).
- Vd of 6 L/kg (420 L in 70 kg adult) exceeds total body water (42 L) — proves binding to tissue proteins and lipid stores, not plasma proteins.
- High Vd drugs (digoxin, chloroquine, amiodarone) — accumulate in tissues, have long half-lives, and require loading doses in Indian clinical practice.
- Plasma protein binding alone produces Vd ≤ 0.05 L/kg — maximum theoretical limit when drug is 100% bound to plasma proteins.
Mnemonics
Vd Interpretation Rule Vd < 0.05 L/kg = Plasma only (heparin). Vd 0.05–0.5 L/kg = Plasma + some tissue. Vd > 1 L/kg = Extensive tissue sequestration. Vd >> body water = Lipophilic + tissue binding (digoxin, chloroquine). High Vd Memory Hook Think: 'Big Vd = Big hiding place in tissues.' A drug with Vd of 6 L/kg is 'hiding' in tissues, not floating in blood. This is why digoxin toxicity is treated with Fab fragments (bind free drug in plasma) and why dosing must account for tissue stores.
NBE Trap
NBE pairs 'plasma protein binding' with high Vd to trap students who confuse protein binding (which limits Vd to ~0.05 L/kg) with tissue distribution (which produces Vd >> body water). Students must recognize that a Vd of 6 L/kg is mathematically impossible with plasma binding alone.
Clinical Pearl
In Indian clinical practice, drugs like chloroquine (used for malaria prophylaxis) and digoxin (used for heart failure) have very high Vd and accumulate in tissues over time. This is why loading doses are essential and why toxicity can persist even after stopping the drug—the tissue reservoir continues to release drug into plasma. Understanding Vd guides dosing adjustments in renal/hepatic disease.
_Reference: KD Tripathi Pharmacology Ch. 1 (Pharmacokinetics); Harrison Principles of Internal Medicine Ch. 5 (Drug Pharmacokinetics)_