A 30 year old female presents with bluish black discoloration of sclera and pinna for last 4 months. Her urine turns black on standing. Which of the following enzyme is most likely to be deficient?

The metabolic defect in alkaptonuria is a defective homogentisate oxidase. 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