This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Tryptophan metabolism detail, Subject name: Biochemistry
Hanup disease is mainly due to: 
Select and explain. 
A. Niacin deficiency, B. Defective transpo of tryptophan, C. Defective serotonin biosynthesis, D. Thiamine deficiency
B: Defective transpo of tryptophan
Answer: B: Defective transpo of tryptophan 
Explanation: Hanup's disease is an autosomal recessive disorder, in which there is: * Defect in neutral amino acid transpoer (tryptophan) * Failure to absorb tryptophan from intestine and also reabsorb it from kidneys =Clinical feature: Aminoaciduria i.e. tryptophan in urine Pellagra (niacin deficiency ) like symptoms (neurological and dermatologic), despite an adequate intake of both tryptophan and niacin. NORMAL AND ABNORMAL ROUTES OF TRYPTOPHAN METABOLISM