A 6-year-old boy is taken to a paediatrician because he has developed pain in his right hip. On physical examination, the doctor feels a large mass near the iliac crest. Plain X-ray films demonstrate a large lytic lesion of the ilium. MRI studies show that the tumor appears to arise in the bone, but extends into the adjacent soft tissues. A large incisional biopsy demonstrates a tumor composed of sheets of small, round, blue cells. Which of the following is the most likely diagnosis?

Ewing sarcoma is the second most common type of bone tumor (after osteosarcoma) in children and adolescents. The tumor is usually a lytic bone lesion, and often both invades the medullary cavity of the bone and extends into extraosseous tissues. Grossly, the tumor is often tan-white in color and shows focal areas of hemorrhage and necrosis. Microscopically, it is composed of sheets of small, round cells with scanty cytoplasm that may appear clear because of their glycogen content. Chondrosarcoma is composed of malignant hyaline and myxoid cailage. Giant cell tumor of bone contains multinucleated giant cells in a background of mononuclear stromal cells. Malignant fibrous histiocytoma has a background of spindled fibroblasts in a storiform pattern admixed with bizarre, multinucleated tumor giant cells.