A 54 year old man presents to a physician with headaches. A complete blood count demonstrates a hematocrit of 62%. The peripheral smear showed normocellular erythrocytes, with increased reticulocytes and nucleated red cells. Bone marrow biopsy demonstrates increased numbers of erythrocytic precursors. Cancer of which of the following organs would be most likely to cause these findings?

The hematologic finding is erythrocytosis, which can be caused by abnormal erythropoietin secretion by renal cell carcinoma (i.e., a paraneoplastic syndrome). Absolute erythrocytosis also occurs in a number of other conditions, such as hypoxia, other types of renal disease, some tumors (e.g., hepatocellular carcinoma, meningioma, pheochromocytoma, cerebellar hemangioblastoma, adrenal adenoma), androgen therapy, Bater's syndrome, or in polycythemia vera. Cancers of the colon , prostate and thyroid do not usually produce inappropriate hormones. Cancers of the ovary can produce male or female sex steroids, but do not produce erythropoietin. Ref: Wyatt C., Butterwoh IV J.F., Moos P.J., Mackey D.C., Brown T.G. (2008). Chapter 16. Pathology of the Kidney and Bladder. In C. Wyatt, J.F. Butterwoh IV, P.J. Moos, D.C. Mackey, T.G. Brown (Eds), Pathology: The Big Picture.