**Core Concept**
Hartnup disorder is a rare genetic disorder that affects the absorption of certain amino acids in the small intestine. It is characterized by a defect in the transport of neutral amino acids, leading to increased excretion of these amino acids in the urine. The disorder is caused by mutations in the SLC6A19 gene, which encodes a neutral amino acid transporter.

**Why the Correct Answer is Right**
In Hartnup disorder, the impaired absorption of neutral amino acids leads to a secondary increase in the excretion of tryptophan, which is a precursor to serotonin. As a result, patients with Hartnup disorder often exhibit symptoms of niacin deficiency, including pellagra, despite adequate dietary intake of tryptophan. This is because tryptophan is converted to niacin in the body, and the impaired absorption of tryptophan results in a reduced availability of niacin.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased absorption of tryptophan - This is incorrect because Hartnup disorder is characterized by impaired absorption of neutral amino acids, including tryptophan.

**Option B:** Deficiency of SLC6A19 gene - While this is a true statement, it is not the correct answer to the question. The question asks what is seen in Hartnup disorder, not what causes it.

**Option C:** Increased excretion of acidic amino acids - This is incorrect because Hartnup disorder affects the absorption of neutral amino acids, not acidic amino acids.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Hartnup disorder can be confused with other conditions that cause pellagra, such as niacin deficiency. A key distinguishing feature is the presence of a tryptophan-responsive pellagra, which is characteristic of Hartnup disorder.

**Correct Answer:** C. Increased excretion of tryptophan.