Hartnup disease

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Hartnup disease is a rare hereditary disorder that results in a rash and brain abnormalities because tryptophan and certain other amino acids are not well absorbed from the intestine and not well reabsorbed by the kidneys, so the body has inadequate amounts of these substances.
(See also Introduction to Congenital Kidney Tubular Disorders.)
In Hartnup disease, the transport of amino acids in the intestine and kidneys is disordered. Hartnup disease occurs when a person inherits two copies of the abnormal gene for the disorder, one from each parent (see Figure: Non–X-Linked (Autosomal) Recessive Disorders). The defective gene controls the absorption of certain amino acids from the intestine and the reabsorption of those amino acids in the kidneys. Consequently, a person with Hartnup disease cannot absorb amino acids properly from the intestine and cannot reabsorb them properly from tubules in the kidneys.
Excessive amounts of amino acids, such as tryptophan, are excreted in the urine. The body is thus left with inadequate amounts of amino acids, which are the building blocks of protein. With too little tryptophan in the blood, the body is unable to make a sufficient amount of the B-complex vitamin niacin, particularly under stress when more vitamins are needed.