**Core Concept:** Hartnup's disease is a rare inherited disorder affecting the absorption of certain amino acids, vitamins, and purines in the intestine. It is caused by defects in the transport proteins of the intestinal epithelial cells.

**Why the Correct Answer is Right:** In Hartnup's disease, there is a deficiency in the genes encoding for the various transport proteins (P-aminobenzoic acid transporter and nicotinamide transporter) involved in the absorption of certain amino acids (phenylalanine, tyrosine, and tryptophan) and purines (hypoxanthine, guanine, and xanthine) in the intestine. As a result, these substances are not absorbed properly and excreted in the urine.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it does not address the specific issue of amino acid, purine, and vitamin deficiency caused by the transport protein deficiency in Hartnup's disease.
B. This option is incorrect as it does not address the excretion of amino acids and purines in urine due to impaired absorption caused by the transport protein deficiency.
C. This option is incorrect as it does not address the specific deficiency in amino acid, purine, and vitamin absorption caused by the transport protein deficiency in Hartnup's disease.
D. This option is incorrect because it does not address the excretion of amino acids and purines in urine due to impaired absorption caused by the transport protein deficiency.

**Clinical Pearl:** Understanding the pathophysiology of Hartnup's disease helps in recognizing the symptoms and signs of the disease, such as hyperuricemia, hyperphenylalaninemia, and hypotaurinuria. Early diagnosis and treatment can prevent complications related to vitamin deficiencies and neurological abnormalities.

**Correct Answer:** D. Hypotaurine, as it is excreted in the urine due to the impaired absorption of sulfur-containing amino acids (methionine and cysteine) caused by the transport protein deficiency in Hartnup's disease. The deficiency leads to the accumulation of sulfur-containing amino acids, which are precursors of hypotaurine, in the blood. These substances are then excreted in the urine, resulting in hypotaurinuria.