<prompt> Darkening of urine on standing is associated withAlkaptonuriaCystinuriaFabrys diseaseTyrosinemia  <response>: Alkaptonuria is caused by defect of the enzyme homogentisate 1,2-dioxygenasehomogentisic acid oxidase.The disorder comes to attention due to change in colour of urine to brownish black staining of diapers.The urine becomes dark on standing,especially if the pH of urine is alkaline,due to the presence of homogentisic acid.Excessive urine homogentisate results in positive reducing substances. Reference:Essential pediatrics-Ghai,8th edition,page no:653. 