Apparent volume of distribution (aVd) is more than total body fluid if drug is:
**Core Concept**
Apparent volume of distribution (aVd) is a pharmacokinetic parameter that estimates the volume of the body in which a drug is distributed. It is calculated by dividing the dose of the drug by its plasma concentration. A high aVd indicates that the drug is distributed throughout the body, whereas a low aVd suggests that the drug is mainly confined to the plasma.
**Why the Correct Answer is Right**
The correct answer is B. Sequestered in tissues. When a drug is sequestered in tissues, it is distributed throughout the body, but its concentration in the plasma is lower due to the binding of the drug to tissue receptors or other molecules. This results in a higher aVd value, as the total body content of the drug is higher than the volume of the plasma. The binding of the drug to tissue receptors can be non-specific or specific, depending on the nature of the drug and the tissue. This phenomenon is observed in many drugs, including local anesthetics, which bind to nerve tissue and are therefore sequestered in tissues.
**Why Each Wrong Option is Incorrect**
**Option A:** Poorly soluble drugs tend to have a low bioavailability and may not be well-absorbed from the gastrointestinal tract, but this does not directly affect the aVd.
**Option C:** Slow elimination of a drug does not necessarily affect its distribution and therefore does not increase the aVd.
**Option D:** Poorly plasma protein-bound drugs may have a lower aVd, as they are more concentrated in the plasma due to their binding to plasma proteins.
**Clinical Pearl / High-Yield Fact**
A high aVd is often seen in lipophilic drugs, which tend to distribute into tissues and fat. This is why drugs like diazepam and lidocaine have a high aVd, as they are lipophilic and tend to bind to fatty tissues.
**✓ Correct Answer: B. Sequestered in tissues**