<prompt> Xanthurenic acid is produced inTyrosine metabolismTryptophan metabolismCysteine metabolismValine metabolism  <response>: In the metabolism of Tryptophan, Kynurenin is an intermediate step. The accumulated Kynurenine is shunted to Xanthurenic acid. So, in pyridoxal deficiency, Xanthurenic acid is excreted in the urine.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 240 