A 55-year-old man presents with a deep venous thrombosis. His full blood count shows a hemoglobin of 18.3 g/dL (normal 13.5-17.5), a white cell count of 8 x 109/L (normal 3.9- 10.0 x 109) and a platelet count of 200 x 109/L (normal 150-400 x 109) This hematological picture is most likely to be associated with: