If urine sample darkens on standing, the most likely condition is :
Alkaptonuria a condition d/t defective homogentisate 1, 2- dioxygenase (1/t defective tyrosine degradation) usually presents after age of 40 with characteristic triad of Homogentisic aciduria (which blacken/darkens on exposure to air /standing and is strongly Benedict's positive), Ochronosis (i.e black pigmentation of cailage & collagenous connective tissue), and Ahritis of large joints
Phenylketonuria
Alkaptonuria
Maple syrup disease
Tyrosinemia