A previously healthy 12-year-old girl presents to the emergency department with a 1-week history of painful swelling of her right upper back. The swelling started as a tennis-ball-sized mass, which progressively enlarged over 1 week and is currently causing discomfort during movement of her neck and right arm. She denies any numbness, tingling, or decreased range of motion. She denies any trauma but after further questioning admits to recently wearing a heavy purse on her right shoulder. She denies any fevers, rashes, recent viral illnesses, or changes in skin color or temperature of the area. Her mother has given her nonsteroidal anti-inflammatory drugs (NSAIDs) and applied warm compresses to the area with no improvement.

Her medical history is remarkable for a neurosurgical intervention at 5 months old for a subcutaneous scalp mass that was removed and diagnosed as a cranial myofibroma, with no further recurrences or sequelae. She also had...

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