Osteosarcoma Codman Sunburst

A 15-year-old boy presents with 3 months of progressive pain and a firm, warm swelling over the distal right thigh, worst at night. Examination reveals restricted painful knee flexion and dilated superficial veins. AP and lateral radiographs of the distal femur show an aggressive lesion. The structure marked **A** in the diagram—a mixed sclerotic and lytic metaphyseal lesion with an aggressive cloud-like osteoid matrix and wide zone of transition—is most consistent with which diagnosis?