This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
1st product of tryptophan Catabolism 
Select and explain. 
A. Kynurenine, B. Xantheurenic acid, C. Bradykinin, D. Melatonin
A: Kynurenine
Answer: A: Kynurenine 
Explanation: Kynurenine is formed from tryptophan in presence of enzyme tryptophan pyrrolaseRef: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 239, fig 18.8