Alkaptonuria

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Summary of Alkaptonuria

Alkaptonuria is a rare autosomal recessive genetic disorder in which the body cannot process the amino acids phenylalanine and tyrosine, leading to accumulation of an intermediate substance called homogentisic acid in the blood and tissues. The accumulating homogentisic acid causes unusually dark urine, damage to cartilage (ochronosis, leading to osteoarthritis) and heart valves as well as precipitating as kidney stones and stones in other organs, including the skin.

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Alkaptonuria

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Alkaptonuria

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Alkaptonuria is an amino acid metabolism disorder characterized by (enzyme) deficiency.

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A 23-year-old man comes to the office because of pain in his left shoulder. He says that this pain started 3 years ago and has progressively worsened. He denies joint trauma, fever, dysuria or morning stiffness. He says that his urine turns black after it is exposed to air since childhood. He has one sexual partner and they regularly use condoms. His temperature is 37.2°C (98.96°F), pulse is 72/min, respirations are 18/min, and blood pressure is 135/80 mmHg. Physical examination shows bilateral scleral darkening and point tenderness upon palpation of his right elbow, left knee and shoulder Leukocyte count is 6,000/mm³. An amino acid catabolism enzyme deficiency is suspected. Which of the following enzymes is most likely deficient?

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