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 in the urine is suggestive of deficiency of which vitamin? 
Select and explain. 
A. Vitamin B6, B. Vitamin B12, C. Folic acid, D. Thiamine
A: Vitamin B6
Answer: A: Vitamin B6 
Explanation: Xanthurenic acid is formed in the side reaction of kynurenine. Kynurenine which is a intermediate of tryptophan metabolism requires vitamin B6 dependent enzyme kynureinase for its normal metabolism. In pyridoxine(Vit B6) deficiency kynurenine is metabolized to xanthurenic acid which is excreted unchanged in urine. Ref: Textbook of Biochemistry DM Vasudevan, 5th Ed, page 208