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
Xanthurenic acid is the metabolic end product of? 
Select and explain. 
A. Xanthine, B. Uric acid, C. Tryptophan, D. Uronic acid
C: Tryptophan
Answer: C: Tryptophan 
Explanation: In the major catabolic pathway of tryptophan, kynurenine is an enzyme dependent on pyridoxal phosphate. Therefore in Vitamin B6 deficiency, the pathway at this level is blocked. The accumulated kynurenine is shunted to Xanthurenic acid and is excreted in urine. Ref: Textbook of Biochemistry DM Vasudevan, 5th Ed, page 208