This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Disorder of Metabolism & Connective Tissue, Subject name: Medicine
Urine turns dark on standing in: (Repeat) 
Select and explain. 
A. Alkaptonuria, B. Maple syrup urine disease, C. Phenyl ketonuria, D. Acute intermittent porphyria
A: Alkaptonuria
Answer: A: Alkaptonuria 
Explanation: Ans: A (Alkaptonuria) Ref: Harrison's Principles of Internal Medicine 18th edition & Harper's illustrated Biochemistry, 28th edition. Chapter 29, pp 254)Explanation:AlkaptonuriaDue to deficiency in homagentisic acid oxidaseResults in the accumulation of homogentisic acidThe illness has three characteristic symptoms;Homogentisicaciduria (the patient's 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 tOochronsis)Diets 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.