Answer: A

Histology: Focally, Multiple large irregular nests are irregularly present amongst large bundles of muscularis propria without prominent desmoplasia. Cytologically, the nests are totally bland, with the cells identical to normal urothelium without appreciable mitotic activity. Focally, there is a detached fragment of non-invasive low grade papillary urothelial carcinoma.

Discussion
In contrast to invasive low grade papillary urothelial carcinoma and why nested carcinoma can be difficult to recognize as cancer is that the nested variant of urothelial carcinoma typically lacks an inflammatory or desmoplastic stromal reaction, usually does not have an overlying papillary component, lacks more irregular shaped nests, and does not consist of small clusters of cells with retraction artifact. Desmoplasia is also unusual may be present in large nested urothelial carcinomas, and is of further value in confirming invasion when present. In contrast to an inverted growth pattern of noninvasive urothelial carcinoma, the presence of urothelial nests, regardless of how bland tumor between muscularis propria is virtually diagnostic of muscle invasive carcinoma. Similarly, florid proliferation of von Brunn nests would cluster under the urothelium as regular evenly-spaced nests with an even non-infiltrative base and would never be present in muscularis propria. Except for urachal rests and the intramural segment of the ureter, benign urothelial proliferations or lesions do not involve the muscularis propria. Nested urothelial carcinomas are frequently muscle invasive and, despite their innocuous histology, are paradoxically associated with aggressive clinical outcome including metastasis and death. Even with therapy, 70% of patients with adequate follow-up die within 4 to 40 months of diagnosis. However stage for stage their prognosis is similar to invasive high grade urothelial carcinoma, with their poor prognosis reflecting typical presentation at high stage.