<prompt> If urine sample darkens on standing: the most likely conditions is -PhenylketonuriaAlkaptonuriaMaple syrup diseaseTyrosinemia  <response>: Ans. is b Le.t Alkaptonuria Alkaptonuriao It is due to deficiency of homogentisate oxidase As a result homogentisic acid homogentisate is excreted execessively in urine. There are three important characteristic features in alkaptonuria-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 imparts a characteristic black-brown colour to urine.Alkapton deposition occurs in sclera, ear. nose, cheeks and intervertebral disc space. A condition called ochronosis. There may be calcification of intervertebral discs.Onchronosis arthritis affecting shoulder, hips, knee.o Benedicts test is strongly positive in urine and so is the ferric chloride FeCl3 test. Benedicts reagent gives a greenish brown precipitate with brownish black supematent. Fehlings reagent FeCl3 gives blue green colour. 