**Core Concept**
Hartnup's disease is a rare genetic disorder affecting the metabolism of certain amino acids, leading to impaired renal reabsorption and excessive urinary excretion of one specific amino acid. This condition is characterized by a mutation in the SLC6A19 gene, encoding a neutral amino acid transporter in the kidneys and intestines.

**Why the Correct Answer is Right**
The amino acid excreted in Hartnup's disease is tryptophan, which is a neutral amino acid that cannot be properly reabsorbed in the kidneys due to the mutated transporter. As a result, tryptophan is excessively excreted in the urine, leading to a deficiency of niacin (vitamin B3), which is synthesized from tryptophan in the body. The impaired tryptophan metabolism also affects the production of serotonin and other tryptophan-derived neurotransmitters.

**Why Each Wrong Option is Incorrect**
**Option A:** Taurine - Taurine is a conditionally essential amino acid, but it is not the primary amino acid excreted in Hartnup's disease.
**Option B:** Glutamine - Glutamine is an essential amino acid, but it is not directly related to Hartnup's disease.
**Option C:** Glycine - Glycine is a non-essential amino acid, but it is not the primary amino acid excreted in Hartnup's disease.

**Clinical Pearl / High-Yield Fact**
Hartnup's disease is a classic example of a genetic disorder affecting amino acid metabolism, highlighting the importance of renal reabsorption mechanisms in maintaining proper amino acid homeostasis.

**Correct Answer:** D. Tryptophan.