A 6-month-old girl presents to her pediatrician with marked left thigh swelling approximately 1 month after receiving her routine 6-month vaccines. The area is warm and tender to touch, and there is no bruising or discoloration. She is treated with a 10-day course of clindamycin, after which the swelling improves somewhat but there remains residual asymmetry and tenderness to the affected area; no other medications are administered during this time. The following week, a magnetic resonance image of the thigh is obtained and demonstrates a hematoma within and adjacent to the thigh musculature with no bone or joint involvement. Her pediatrician orders initial screening laboratory tests, including a complete blood cell count (CBC), prothrombin time (PT), and activated partial thromboplastin time (aPTT). Three days later, she presents to the emergency department with significant right upper extremity swelling, bruising, pain, and limited range of motion at the site of the venipuncture...

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