<prompt> Tryptophan loading test is done to detect deficiency of:TPPPLPBiotinFolate  <response>: Tryptophan loading test detects xanthurenic acid in urine which indicates deficiency of Vitamin B6 i.e. PLP Pyridoxal phosphate. In the absence of Vitamin B6 the excess of 3-OH-Kynurenine is conveed to Xanthurenic acid after given a load of tryptophan see Fig below Figure: Tryptophan catabolism pathway Note:- Histidine load test detects FIGLU Form-Imino Glutamate in urine which indicates deficiency of Vitamin B9 Deficiency Common metabolite Specific metabolite Vitamin B6 Homocysteine Xanthurenic acid Vitamin B9 Homocysteine FIGLU Vitamin B12 Homocysteine L-methyl malonic acid 