A 4 year-old boy presents to the emergency room with a chief complaint of worsening limp. His parents tell you that yesterday he complained of pain in his left leg, and today he refuses to bear weight. On exam, he is febrile to 39º Celsius, tachycardic and appears ill. Exam is significant for exquisite tenderness over the left distal femur. There is no redness or swelling of the left knee. His labs are significant for WBC of 14, CRP of 150, ESR of 90. Blood cultures are drawn. X-ray of the femur reveals non-specific swelling around the femur, but no fracture. MRI shows:


What is the Diagnosis?


Image: Case courtesy of Dr Mohammad A. ElBeialy, Radiopaedia.org. From the case rID: 38589