This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins and Minerals, Subject name: Biochemistry
Pellagra is caused by - 
Select and explain. 
A. Hartnup's disease, B. Cystinuria, C. Cystinosis, D. Type I Tyrosinemia
A: Hartnup's disease
Answer: A: Hartnup's disease 
Explanation: Ans. is 'a' i.e., Hartnup's diseaseHartnup's disease* It is a defect of neutral aminoacid transporter which is specific for tryptophan. This transporter is responsible for tryptophan absorption along intestine as well as tryptophan reabsorption across renal tubules.* When tryptophan is not absorbed, tryptophan can not be converted into niacin (Niacin is endogenously synthesised from tryptophan). Hence it presents as pellagra.* As the transporter is also related to tryptophan reabsorption along renal tubules, tryptophan will be lost in the urine. Hence it presents as aminoaciduria.