In Hartnup disorder, metabolism of which of the following amino acids is affected?
**Core Concept**
Hartnup disorder is a genetic disorder characterized by impaired absorption and metabolism of neutral amino acids, leading to various clinical manifestations such as pellagra-like symptoms, dermatitis, and neurological abnormalities.
**Why the Correct Answer is Right**
The affected amino acid in Hartnup disorder is tryptophan, which is a neutral amino acid. The disorder is caused by mutations in the SLC6A19 gene, which encodes a neutral amino acid transporter responsible for the reabsorption of tryptophan in the kidneys. The impaired reabsorption of tryptophan leads to its excessive excretion in the urine, resulting in a tryptophan deficiency. Since tryptophan is a precursor to niacin, its deficiency can lead to niacin deficiency and subsequent development of pellagra-like symptoms.
**Why Each Wrong Option is Incorrect**
**Option A:** Phenylalanine - While phenylalanine is an amino acid, it is not the primary amino acid affected in Hartnup disorder. Phenylalanine is a non-neutral amino acid and is primarily affected in phenylketonuria (PKU), a different genetic disorder.
**Option B:** Leucine - Leucine is a branched-chain amino acid and is not the primary amino acid affected in Hartnup disorder. Leucine metabolism is impaired in maple syrup urine disease, a different genetic disorder.
**Option C:** Methionine - Methionine is a sulfur-containing amino acid and is not the primary amino acid affected in Hartnup disorder. Methionine metabolism is impaired in homocystinuria, a different genetic disorder.
**Clinical Pearl / High-Yield Fact**
Hartnup disorder is a rare genetic disorder but should be considered in the differential diagnosis of patients presenting with pellagra-like symptoms, particularly those with a family history of the disorder.
**Correct Answer: C. Tryptophan**