**Core Concept**
Hartnup disease is a rare genetic disorder affecting the absorption of certain amino acids, specifically those that are neutral or uncharged at physiological pH. This condition is caused by mutations in the SLC6A19 gene, leading to impaired function of the neutral amino acid transporter in the small intestine.

**Why the Correct Answer is Right**
The limiting amino acid in Hartnup disease is tryptophan, which is an essential amino acid that plays a crucial role in the synthesis of serotonin and niacin. In Hartnup disease, the impaired absorption of tryptophan leads to its decreased availability for these pathways, resulting in symptoms such as dermatitis, ataxia, and photosensitivity. The mutation in the SLC6A19 gene affects the transport of tryptophan and other neutral amino acids across the intestinal epithelium, leading to their excessive excretion in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, so we cannot evaluate its correctness.
**Option B:** While other amino acids may be affected in Hartnup disease, tryptophan is the primary limiting amino acid due to its essential role in the body.
**Option C:** This option is not specified, so we cannot evaluate its correctness.
**Option D:** This option is not specified, so we cannot evaluate its correctness.

**Clinical Pearl / High-Yield Fact**
Hartnup disease is characterized by the excessive excretion of tryptophan in the urine, which can be detected using a simple test. This fact can help clinicians diagnose the condition and initiate appropriate treatment.

**Correct Answer:** B. Tryptophan