A 6-year-old girl from Mumbai presents with a 2-week history of abdominal distension, vomiting, and constipation. Physical examination reveals a firm, non-tender abdominal mass in the right lower quadrant, and mild hepatomegaly. Laboratory findings: Hb 9.8 g/dL, WBC 6,500/µL (normal differential), platelets 180,000/µL. Abdominal ultrasound shows a large retroperitoneal mass (8 cm × 6 cm) with areas of necrosis and calcification, compressing the right ureter. Urine shows microscopic hematuria. A biopsy of the mass reveals small, round, blue cells with high nuclear-to-cytoplasmic ratio, scant cytoplasm, and numerous mitotic figures. Immunohistochemistry: CD99+, FLI1+, desmin−, myogenin−. What is the most likely diagnosis?