What condition results in too many erythrocytes, making the blood thick?

Polycythemia vera is a myeloproliferative neoplasm characterized by the overproduction of erythrocytes (red blood cells), leading to an increase in blood viscosity or thickness. This condition typically arises from mutations, commonly in the JAK2 gene, which enhance cell proliferation in the bone marrow. The excess erythrocytes can result in various complications, such as increased risk for clotting and vascular problems due to the thickened blood.

In contrast, hemophilia is a bleeding disorder resulting from deficiencies in clotting factors, leading to difficulty in blood coagulation, rather than an increase in cell counts. Thrombocytopenia is characterized by a reduced number of platelets, which can also lead to bleeding issues but not to the thickening of blood. Anemia, on the other hand, is defined by a deficiency of red blood cells or hemoglobin, leading to thinner blood and decreased oxygen-carrying capacity, opposite to what happens in polycythemia vera.