You are doing a newborn exam on a baby delivered by forceps and notice a finding that you have trouble describing. On the head, you notice a "bump." On palpation, it appears to be contained within a "compartment" though you're not sure what that really means. It is a fluctuant mass. Here's an X-ray.
The mass (white arrow) does not cross the point indicated by the black arrow.
Challenge: What's the diagnosis?
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