Answer: A

Histologic Description: This is an encapsulated neoplasm which is epithelial predominant. Most of the neoplasm consists of tubules lined by primitive columnar cells with mitotic activity, diagnostic of epithelial Wilms tumor. Of interest, there are areas at the edge of this encapsulated neoplasm where the cells are better differentiated, more cuboidal, less mitotically active, and associated with sclerotic stroma and psammomatous microcalcifications. These areas resemble metanephric adenoma. It is not uncommon for epithelial Wilms tumors to have areas within them that resemble metanephric adenoma.

Differential Diagnosis: Metanephric adenoma would not have the proliferative primitive areas with columnar nucleoli that are typical of Wilms tumor. Metanephric adenofibroma would feature a spindle cell stroma that is identical to metanephric stromal tumor, along with areas of metanephric adenoma. These lesions also may be associated with papillary renal cell carcinoma (composite metanephric adenofibroma-papillary renal cell carcinoma). The current case lacks the stroma and papillary renal cell carcinoma component of such lesions, and the mitotically active primitive tubules are better for Wilms tumor than metanephric adenoma. Papillary renal cell carcinoma may have a blue cell appearance (so-called type I papillary renal cell carcinoma). However, the nuclei of such lesions are typically more open, and these neoplasms do not label for WT1 like metanephric adenoma and Wilms tumor.