A 27 year old man with AIDS develops a reddish, slightly raised rash on his face, neck, and mouth, consistent in appearance with Kaposi's sarcoma. Microscopically, the proliferating cells in this malignancy most closely resemble which of the following?

Kaposi's sarcoma is a spindle cell neoplasm that is highly associated with AIDS and with the Herpes simplex virus type 8. The tumor has an appearance very similar to that of angiosarcoma-proliferating stromal cells and endothelium creating vascular channels that contain blood cells. Carcinosarcoma is a tumor that contains malignant epithelial cells and malignant stromal cells. There is no epithelial element in Kaposi's sarcoma. Although lymphoma occurs with increased frequency in AIDS, it does not resemble Kaposi's sarcoma. Lymphoma involves neoplastic lymphocytes, whereas Kaposi's sarcoma involves neoplastic vascular structures. Malignant fibrous histiocytoma (MFH) is an extremely poorly differentiated (anaplastic) stromal malignancy. MFH does not produce any recognizable mesenchymal structures- thus, the production of vascular structures by Kaposi's sarcoma differentiates the two tumors. Ref: Wyatt C., Butterwoh IV J.F., Moos P.J., Mackey D.C., Brown T.G. (2008). Chapter 9. Vascular Pathology. In C. Wyatt, J.F. Butterwoh IV, P.J. Moos, D.C. Mackey, T.G. Brown (Eds), Pathology: The Big Picture.