A 10 year old child is suspected of having pellagra because of chronic symptoms including diarrhea, a red scaly rash, and mild cerebellar ataxia. However, his diet is not deficient in protein and he appears to be ingesting adequate amounts of niacin. A sister has a similar problem. Chemical analysis of his urine demonstrates large amounts of free amino acids. Which of the following is the most likely diagnosis?

The child has Hanup's disease. This condition clinically resembles pellagra ("diarrhea, dementia, and dermatitis"), and may be misdiagnosed as this nutritional (niacin) deficiency. In fact, niacin therapy may actually be helpful in controlling the symptoms. The underlying problem is a defect in the epithelial transpo of neutral amino acids, including tryptophan, which can act as a precursor of niacin. The defective amino acid transpo leads to poor absorption of dietary amino acids as well as excess amino acid secretion in the urine. Alkaptonuria is characterized by urine that turns black upon standing and a debilitating ahritis. Carcinoid syndrome is seen in patients with carcinoid tumor. It is characterized by episodes of flushing, diarrhea, hypeension, and bronchoconstriction. Ehlers-Danlos syndrome is a disease characterized by abnormal collagen formation leading to very elastic skin, joint problems, and fragility of some blood vessels and the intestines. Ref: Bender D.A. (2011). Chapter 44. Micronutrients: Vitamins & Minerals. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds),Harper's Illustrated Biochemistry, 29e.