If urine sample darkens on standing: the most likely conditions are
The urine of alkaptonuric patients becomes dark after being exposed to air.
In presence of oxygen, the colorless homogentisate (homogentisic acid) undergoes spontaneous oxidation to yield benzoquinone acetate, which polymerizes to form black-brown pigment alkapton.
Alkapton imparts a characteristic black-brown colour to urine.
Phenylketonuria
Alkaptonuria
Maple syrup disease
Tyrosinemia