**Question:** Loading dose of a drug depends upon

A. Patient's body weight
B. Drug's molecular weight
C. Plasma protein binding
D. Drug's volume of distribution

**Core Concept:** Loading dose is a higher initial dose of a drug given to rapidly achieve therapeutic drug concentration in the bloodstream. This is followed by a maintenance dose to maintain the therapeutic concentration. Loading dose selection depends on various factors related to the drug itself and the patient.

**Why the Correct Answer is Right:**
The correct answer depends on the drug's volume of distribution (D). The volume of distribution is a measure of how much of a drug is distributed into extravascular fluid and tissues compared to the plasma. A larger volume of distribution indicates that the drug will reach deeper tissues and organs, so a larger dose is required to achieve therapeutic concentrations.

**Why Each Wrong Option is Incorrect:**
A. Patient's body weight (A) is not directly related to the drug's pharmacokinetic properties and does not solely determine the loading dose.
B. Drug's molecular weight (B) is related to its solubility and permeability, but not directly related to the loading dose determination.
C. Plasma protein binding (C) is related to the drug's ability to bind to plasma proteins, which can affect its distribution and pharmacokinetics, but it does not solely determine the loading dose.

**Clinical Pearl / High-Yield Fact:**
Drugs with a larger volume of distribution, like those with lipophilic (fat-soluble) properties, typically require a higher loading dose to achieve therapeutic concentrations. Understanding these factors is essential for proper dosing and achieving optimal therapeutic outcomes while minimizing side effects.

**Correct Answer:** D. Drug's volume of distribution