This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Classification and metabolism of amino acids, Subject name: Biochemistry
Which of the following is elevated in Pyridoxal Phosphate deficiency? 
Select and explain. 
A. FIGLU, B. Homocystine, C. Methylmalonic acid, D. Xanthurenic acid
D: Xanthurenic acid
Answer: D: Xanthurenic acid 
Explanation: Xanthurenic acid >> Homocysteine (Both are increased in PLP deficiency) 3-OH Kynurenine is one of the intermediate in Tryptophan catabolism Normally conveed to next intermediate and this conversion requires vitamin B6/ PLP. But in B6 deficiency, 3-OH Kynurenine is diveed to form alternate metabolite= Xanthurenic acid (See fig) which is excreted in urine. Homocystine is a chemical compound consisting of two homocysteine molecules joined by a disulfide.