XDH and xanthinuria type I: Xanthinuria is classified into three types: type I, caused by XOR deficiency; type II, due to mutations affecting both XOR and aldehyde oxidase (AO) [67]; and type III, which involves mutations in molybdenum cofactor sulfurase (MOCOS), impacting XOR, AO, and sulphite oxidase (SO) activity (all molybdenum containing enzymes).