What are the possible causes of urine turning black on standing and exposed to air? provided that the urine was normal colored when passed ?

Genetic disease: Alkaptonuria is a rare inherited disorder. It occurs when your body can’t produce enough of an enzyme called homogentisic dioxygenase (HGD). This enzyme break down a toxic substance called homogentisic acid. When you don’t produce enough HGD, homogentisic acid builds up in your body, the bones & cartilage becomes discolored/brittle, Urine that turns Brown or black when exposed to air