<prompt> Xantheurenic acid is produced in -Tyrosine metabolismTryptophan metabolismCysteine metabolismValine metabolism  <response>: Ans. is b i.e., Tryptophan metabolism o Serotonin secretion is increased in carcinoid syndrome which causes flushingQ. VasoconstrictionQ, diarrheaQ and bronchospasm respiratory distress,Metabolism of tryptophano Tryptophan is an essential amino acidQ, containing indol ringQ. Tryptophan is oxidized to produce alanine which is glucogenic and acetyl-CoA which is ketogenic. Therefore, tryptophan is both glucogenic and ketogenic. Tryptophan is precursor for the biosynthess of niacinQ a vitaminQ, serotonin a neurotransmitter and melatoninQ a hormone.o Tryptophan metabolism occurs by two pathways: i kinurenine pathwayQ and ii serotonine pathway.1 Kynurenine pathway :- Tryptophan is oxidized to kynurenine, which is then converted to either niacin nicotinic acid or acetyl-CoA. In the reaction alanine is also produced. o Deficiency of tryptophan can cause deficiency of niacin if also deficient in diet. This can leads to Pallegra: dermatitis, diarrhea and dementia.o For every 60 mg of tryptophan, 1 mg. equivalent of niacin can be generatedQ.o In vitamin B6 deficiency the metabolism is shuted to xanthurenic acid. Therefore, xantheurenic acid excretion is increased in urine.2 Serotonin pathway:- Tryptophan is first oxidized to serotonine which is further converted to melatonin. 