<prompt> Urine turns dark on standing in: RepeatAlkaptonuriaMaple syrup urine diseasePhenyl ketonuriaAcute intermittent porphyria  <response>: Ans: A Alkaptonuria Ref: Harrisons Principles of Internal Medicine 18th edition Harpers illustrated Biochemistry, 28th edition. Chapter 29, pp 254Explanation:AlkaptonuriaDue to deficiency in homagentisic acid oxidaseResults in the accumulation of homogentisic acidThe illness has three characteristic symptoms;Homogentisicaciduria the patients urine contains elevated levels of homogentisic acid, which is oxidized to a dark pigment on standing,Large joint arthritisBlack ochronotic pigmentation of cartilage and collagenous tissue tOochronsisDiets low in protein--especially in phenylalanine and tyrosine--help reduce the levels of homogentisic acid, and decrease the amount of pigment deposited in body tissues.Although alkaptonuria is not life-threatening, the associated arthritis may be severely crippling. 