**Core Concept**
The distribution of a drug to body fat is influenced by its lipophilicity, which is a measure of its ability to dissolve in fats, oils, and non-polar solvents. Drugs that are highly lipophilic tend to accumulate in body fat, leading to a prolonged duration of action and potential toxicity.

**Why the Correct Answer is Right**
The correct answer is a drug that is highly lipophilic, meaning it has a high affinity for non-polar solvents such as body fat. This property allows the drug to penetrate the lipid bilayer of cells and accumulate in adipose tissue, leading to a prolonged half-life and increased risk of toxicity. The high lipophilicity of this drug also makes it more resistant to metabolism and excretion, further contributing to its accumulation in body fat.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a highly water-soluble drug that is primarily excreted in the urine. Its low lipophilicity makes it less likely to accumulate in body fat.
**Option B:** This option is incorrect because it is a polar drug that is primarily metabolized in the liver. Its high polarity makes it less likely to penetrate cell membranes and accumulate in body fat.
**Option C:** This option is incorrect because it is a drug that is highly protein-bound in the plasma. While protein binding can affect the distribution of a drug, it does not directly influence its accumulation in body fat.

**Clinical Pearl / High-Yield Fact**
Drugs that are highly lipophilic can accumulate in body fat and lead to prolonged toxicity, particularly in cases of overdose or chronic use. This is why certain drugs, such as beta-blockers and benzodiazepines, can have a prolonged duration of action and increased risk of toxicity in patients with high body fat stores.

**Correct Answer: D. A highly lipophilic drug such as diazepam.**