Rhabdomyosarcoma — Sarcoma Botryoides

A 4-year-old boy presents with dysuria, poor urinary stream, and gross hematuria for three weeks. His mother reports passage of reddish mucoid debris in the urine. On examination, a firm suprapubic mass is palpable. Contrast-enhanced CT of the pelvis shows a large lobulated polypoid soft-tissue mass filling the bladder lumen with a characteristic grape-like configuration, as marked **B** in the diagram. The mass shows heterogeneous enhancement without calcification, and there is bladder outlet obstruction with bilateral hydroureteronephrosis. The kidneys and abdominal vessels are spared. Which of the following is the most likely diagnosis of the lesion marked **B**?