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What is Darkening of urine on standing is associated with – and explain in detail?
Alkaptonuria

It is an autosomal recessive disorder.
It is due to deficiency of enzyme homogentisic oxidase.
A large amount of homogentisic acid is accumulated in the body and excreted in urine.

Clinical manifestations

Clinical features consist of -


Ochronosis → Dark spot on the sclera or ear cartilage.
Arthritis → Spine, hip, knee.


But both these occur in adulthood.
The only sign of disease in children is a blackening of urine on standing. This is caused by oxidation and polymerization of homogentisic acid.
High incidences of heart disease (mitral and aortic valvulitis, calcification of heart valves, MI) have been noted.