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For some magical reason, this is a USMLE favorite.
It is caused by a deficiency of homogentisic acid oxidase (homogentisate oxidase), which is an enzyme necessary for the breakdown of tyrosine to fumarate.
The highest yield thing you need to know is that the urine becomes dark/black upon standing, or with the addition of citrate. They will mention this in probably ~80% of alkaptonuria questions.
However ~20% of the time, they won’t mention that the urine turns dark. They’ll instead say that the patient has athralgias (painful joints), where biopsy shows blue/black/brown connective tissue (ochronosis). Homogentisate buildup in joints is toxic, which is why it causes discoloration and pain (ochronotic arthropathy). Rarely you might see a question that mentions brownish discoloration of the sclera.
| Dude with urine that turns black on standing. This is caused by a defect in breakdown of which amino acid? Tyrosine
Dude has joint pain and blue/grey discoloration of the pinna of his ear and sclerae. Which enzyme is deficient? Homogentisic acid oxidase |
That’s all you need to know about alkaptonuria.
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