**Core Concept**
Hartnup's disease is a rare genetic disorder involving an inborn error of metabolism. It affects the body's ability to absorb certain amino acids from the diet. The underlying issue is related to the transport of neutral amino acids across the intestinal mucosa and renal tubules.

**Why the Correct Answer is Right**
The correct answer is related to the metabolism of **tryptophan**, an essential amino acid. In Hartnup's disease, the metabolism of tryptophan is disturbed due to impaired transport of neutral amino acids. This leads to reduced absorption of tryptophan and other neutral amino acids, resulting in various clinical manifestations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Hartnup's disease is not primarily associated with **disturbed carbohydrate metabolism**.
**Option B:** This option is incorrect as Hartnup's disease does not directly involve **lipid metabolism**.
**Option C:** This option is incorrect because, although **vitamin metabolism** can be indirectly affected, it is not the primary issue in Hartnup's disease.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Hartnup's disease can lead to **niacin deficiency**, as tryptophan is a precursor to niacin. This deficiency can cause pellagra-like symptoms.

**Correct Answer:** D. tryptophan