This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Blackening of urine on exposure to atmosphere is observed in: 
Select and explain. 
A. Phenylketonuria, B. Maple syrup urine disease, C. Hanup disease, D. Alkaptonuria
D: Alkaptonuria
Answer: D: Alkaptonuria 
Explanation: Alkaptonuria/Black urine disease/AlcaptonuriaIt is a rare inherited genetic disorder of phenylalanine and tyrosine metabolism.This is an autosomal recessive condition that is due to a defect in the enzyme homogentisate 1,2-dioxygenase, which paicipates in the degradation of tyrosine.As a result, homogentisic acid and its oxide, called alkapton, accumulate in the blood and are excreted in urine in large amounts (hence -uria).Excessive homogentisic acid causes damage to cailage (ochronosis, leading to osteoahritis) and hea valves as well as precipitating as kidney stones.Treatment with nitisinone, which suppresses homogentisic acid production, is being studied