Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Volume of distribution of drug is expressed in Litre
Reason (R) : Volume of distribution is directly proportional to plasma drug concentration
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true and (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

The apparent volume of distribution (\(V_d\)) does not represent a real anatomical volume.
Highly lipid-soluble drugs can have a \(V_d\) much larger than the animal's total body volume because their plasma concentration (\(C_p\)) is extremely low.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The apparent volume of distribution (\(V_d\)) is a theoretical pharmacokinetic parameter that relates the total amount of drug in the body to its concentration in the plasma.
Key Formula or Approach:
The relationship is defined by the following pharmacokinetic equation:
\[ V_d = \frac{A}{C_p} \]
where:
- \( A \) is the total amount of drug in the body (dose).
- \( C_p \) is the plasma drug concentration.

Step 2: Detailed Explanation:
Let us evaluate both statements:
- Assertion (A): The volume of distribution is a theoretical volume.
It is expressed in units of volume, such as Liters (L), or normalized to body weight as Liters per kilogram (L/kg). Thus, Assertion (A) is correct.
- Reason (R): According to the equation \( V_d = \frac{A}{C_p} \), the volume of distribution is inversely proportional to the plasma drug concentration (\(C_p\)).
If a drug remains largely bound to plasma proteins within the vascular space, \(C_p\) is high and \(V_d\) is small.
If a drug is highly lipid-soluble and distributes widely into tissues, \(C_p\) is low and \(V_d\) is large.
Therefore, \(V_d\) is inversely proportional, not directly proportional, making Reason (R) false.

Step 3: Final Answer:
Assertion (A) is true but Reason (R) is false. This corresponds to option (C).