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.