This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Which of the following is the 1st product of tryptophan catabolism? 
Select and explain. 
A. Xantheurenate, B. Kynurenine, C. PAF, D. Bradykinin
B: Kynurenine
Answer: B: Kynurenine 
Explanation: Tryptophan is degraded to amphibolic intermediates the kynurenine-anthranilate pathway. Tryptophan oxygenase conves tryptophan to N formyl kynurenine by opening the indole ring and by incorporating molecular oxygen. This inturn is catalysed by kynurenine formylase to produce kynurenine. Hanup's disease result from impaired intestinal and renal transpo of tryptophan and other neutral amino acids. This defect accounts for deficient niacin biosynthesis resulting in pellagra like signs and symptoms. Ref: Harper's Illustrated Biochemistry, 29e, Chapter 29