A 14-year-old girl from Mumbai presents with a 6-week history of progressive left hip pain and a limp. She denies any history of trauma. On examination, she has restricted hip abduction and internal rotation. A plain radiograph of the pelvis shows a large lytic lesion in the left ilium with cortical destruction and a soft tissue mass. MRI reveals a heterogeneous mass with areas of necrosis and hemorrhage, and the tumor crosses the physeal line. Serum LDH is markedly elevated at 1200 U/L. A biopsy shows small round cells with immunohistochemical positivity for CD99 and FLI1. What is the most likely diagnosis?