**Core Concept**
Hartnup's disease is a rare genetic disorder caused by a mutation in the SLC6A19 gene, leading to impaired absorption of neutral amino acids in the small intestine. This results in a complex interplay of metabolic and clinical manifestations.

**Why the Correct Answer is Right**
Hartnup's disease is characterized by a combination of features including:
* Pellegra-like symptoms (such as dermatitis, diarrhea, and dementia) due to niacin deficiency
* Ataxia and tremors due to tryptophan deficiency and subsequent serotonin synthesis impairment
* Photosensitivity and skin rashes
* Mental changes and personality disorders

The correct answer is one of the options listed below.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it's actually a feature of Hartnup's disease. The disorder is indeed associated with impaired tryptophan absorption, leading to serotonin synthesis impairment and subsequent ataxia and tremors.
**Option B:** This option is incorrect because it's also a feature of Hartnup's disease. The disorder is indeed characterized by photosensitivity and skin rashes.
**Option C:** This option is incorrect because it's a feature of Hartnup's disease. The disorder is indeed associated with pellegra-like symptoms due to niacin deficiency.
**Option D:** This option is incorrect because it's a feature of Hartnup's disease. The disorder is indeed associated with mental changes and personality disorders.

**Clinical Pearl / High-Yield Fact**
Hartnup's disease is often misdiagnosed as multiple sclerosis due to its complex and varied clinical presentation. It's essential to consider this condition in patients with unexplained neurological symptoms and a history of photosensitivity.

**Correct Answer: A. Ataxia is a feature of Hartnup's disease.**