Wilms Tumor

A 4-year-old boy is brought to the pediatric emergency department by his mother who noticed a large, firm, painless abdominal mass while bathing him. Imaging confirms an intrarenal origin with a well-circumscribed, encapsulated mass. The structure marked **B** in the diagram is suspected. Which of the following is the most characteristic histological feature of this tumor that would be documented in the pathology report?

A 3-year-old child presents with a painless palpable abdominal mass discovered by the mother during bathing. On examination, the pediatrician notes a smooth, non-tender flank mass that does NOT cross the midline, mild hypertension, and gross hematuria. Physical examination also reveals aniridia, hemihypertrophy, and subtle genitourinary anomalies. Contrast-enhanced CT abdomen shows a large heterogeneous intra-renal mass with the classic "claw sign" of normal renal parenchyma stretched around the tumor margin. The structure marked **A** in the diagram is most consistent with which of the following clinical and pathological features?