A 30 year old female presents with bluish black discoloration of sclera and pinna for last 4 months. Her urine turns black on standing. What is the most likely diagnosis?
Clinical Presentation of Alkaptonuria:

Normal Life till 3rd or 4th decade.
Urine Darkness on standing is the only manifestation in children.
In adults ochronosis, i.e. Alkaptone body deposited in intervertebral disk, cartilage of nose, pinna, etc. leading to pigmentation.
Arthritis 
No mental retardation

Laboratory Diagnosis:

Alkalanization increase darkening of urine.
Benedict's test positive in urine because homogentisic acid is reducing agent.
Ferric chloride test positive.
Silver nitrate test positive.

Reference- Harper’s illustrated biochemistry. 30th edition page -304
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Alkaptonuria
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