Alkaptonuria
General: An autosomal recessive disorder of amino acid metabolism, caused by deficiency of homogentisate 1,2-dioxygenase, located on chromosome 3q2. Homogentisic acid is a metabolite of tyrosine and phenylalanine; it is secreted in the urine, turning it black, and its accumulation (as polymerized homogentisic acid) leads to black discoloration of heart valves, healed infarcts, arterial walls, atheromata, and joint connective tissue.
Gross: Blue-black discoloration of the urine and various connective tissues as described above.
Microscopic: Brown or ochre, sometimes termed "ochronosis," though this is not specific.
Differential Diagnosis:
- Iron overload/deposition
- Homocystinuria: no pigment
Stains:
- Positive: Fontana-Masson
- Negative: Prussian blue
- Suggested, focused panel:
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Images:
Cases:
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