Things that are DARK on US are generally fluid (water, urine, bile, watery pus, CSF, other cystic collections) or flowing blood (clotted blood looks very different). So this dark thing on the image could be any of these, but given its very strange shape, it should make you think of something related to the kidney, rather than just a cyst or an abscess. What part of the kidney could be this shape?

Findings: funny fluid collection

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