A 2-year old male child presents with pellagra like lesions with passage of amino acids in urine. In the family, out of 4 siblings, two are having similar symptoms with two normal siblings and parents are not affected. Most likely diagnosis is:

Ans. d. Hanup disease. Hanup Disease Genetic Defect :A rare autosomal recessive disorder that results in the mutation of a sodium-dependent transpo channel of neutral amino acids (ie, tryptophan). Pathophysiology :The neutral amino acid transpo channel is present in both the proximal tubule of the nephron and the brush border of the small intestine . If this transpo channel is defective, neutral amino acids cannot be absorbed in the intestine or reabsorbed by the kidney after filtration, thereby resulting in a relative deficiency of the neutral amino acids, such as tryptophan . If the body is deficient in tryptophan (a precursor for niacin), symptoms can arise that mimic niacin deficiency (ie, pellagra). Clinical Manifestations Symptoms appear intermittently and tend to decrease with age. Symptoms include a photosensitive dermatitis that affects face, neck, and extensor surfaces of limbs and neurologic signs (headaches; personality disturbances ; photophobia; mental retardation; cerebellar ataxia ). Lab findings: Renal aminoaciduria ; indoles in the urine. Treatment : Nicotinic acid supplements.