Final answer:
The spleen is the organ that becomes enlarged in polycythemia vera due to its role in filtering blood and managing the excess red blood cells produced by the bone marrow.
Step-by-step explanation:
The organ that will be enlarged in polycythemia vera is the spleen. Polycythemia vera is characterized by an excessive production of immature erythrocytes by the bone marrow, which can lead to increased blood viscosity and higher blood pressure, challenging the heart's ability to pump blood effectively. As a result, the spleen, which is involved in filtering blood and removing old or damaged red blood cells (RBCs), becomes overactive and enlarges due to the increased workload of filtering the abnormally high number of RBCs. This condition, known as splenomegaly, can also occur in other disorders, such as myelofibrosis, where inflammation and scar tissue formation in the bone marrow impair normal blood cell production.