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Context.—: Renal malignancies can be divided into cortical- and medullary-based tumors, the latter of which classically infiltrate the renal parenchyma by extending between nonneoplastic structures. Although high-grade cortical tumors can rarely exhibit the same growth pattern, the infiltrative morphology should elicit a differential diagnosis to be considered in each case. However, these diagnoses can be challenging to distinguish, especially on small renal biopsy samples., Objective.—: To provide an overview of the clinical, gross, and microscopic findings; genetic and molecular alterations; and immunohistochemical evaluation of medullary-based renal tumors and other tumor types with overlapping morphologies and growth patterns., Data Sources.—: Literature review and personal observations were used to compile the information in this review., Conclusions.—: Collecting duct carcinoma is a prototypical medullary-based tumor, and although diagnostic criteria exist, it remains a diagnosis of exclusion, especially with ancillary techniques aiding the recognition of established as well as more recently described neoplasms. Other medullary-based malignancies included in the differential diagnosis include renal medullary carcinoma/renal cell carcinoma unclassified with medullary phenotype, fumarate hydratase-deficient renal cell carcinoma, and upper tract urothelial carcinoma. Moreover, other rare entities should be excluded, including metastatic carcinoma, lymphoma, and melanoma. In addition to potential prognostic differences, accurate diagnoses can have important surgical and clinical management implications.

Cytomegalovirus latency, though ubiquitous in the human population, is known to cause colitis in both immunocompromised and immunocompetent hosts. Furthermore, the clinical, endoscopic, and histologic appearance of cytomegalovirus colitis can mimic that of inflammatory bowel disease, an extremely well-documented disease. In this context, though many reports have looked at inflammatory bowel disease with superimposed cytomegalovirus infection, less attention has been paid to cytomegalovirus as a primary cause of isolated colitis. Owing to the rarity of this phenomenon, it is important to consider this diagnosis and implement proper testing to avoid misdiagnosis and mismanagement.