What is a common paraneoplastic syndrome associated with renal cell carcinoma (RCC)?

• A. Hypercalcemia due to bone metastases
• B. Anemia due to marrow infiltration
• C. Hyperkalemia due to kidney dysfunction
• D. Erythrocytosis due to ectopic erythropoietin production

Answer: (D)

RCC can cause systemic effects because tumor cells may secrete erythropoietin, driving increased red blood cell production independent of normal kidney feedback. This leads to erythrocytosis (polycythemia), with elevated hematocrit and hemoglobin—classically described as a paraneoplastic phenomenon of RCC. Hypercalcemia, anemia from marrow infiltration, or hyperkalemia from kidney dysfunction are less characteristic mechanisms for RCC and reflect different processes (bone metastases or direct marrow involvement or renal failure, respectively). Thus, the erythrocytosis from ectopic erythropoietin production best explains a common paraneoplastic presentation in RCC.