<prompt> If urine sample darkens on standing: the most likely conditions is ?PhenylketonuriaAlkaptonuriaMaple syrup diseaseTyrosinemia  <response>: Ans. is b i.e., AlkaptonuriaAlkaptonuriaIt is due to deficiency of homogentisate oxidase. As a result homogentisic acid homogentisate is excreted execessively in urine. There arc three impoant characteristic features in alkaptonuria?i Urine becomes dark after being exposed to air. It is due spontaneous oxidation of homogentisate into benzoquinone acetate, which polymerse to form black-brown pigment alkapton which impas a characteristic black-brown colour to urine.ii Alkapton deposition occurs in sclera, ear, nose, cheeks and interveebral disc space. A condition called ochronosis. There may be calcification of interveebral discs.iii Onchronosis ahritis affecting shoulder, hips, knee.Benedicts test is strongly positive in urine and so is the ferric chloride FeC1 3test. Benedicts reagent gives a greenish brown precipitate with brownish black supernatent. Fehlings reagent FeC13 gives blue green colour. 