What is primary erythrocytosis?

Primary erythrocytosis, also known as polycythemia vera, is defined as an increase in red blood cells (RBCs) that occurs due to an intrinsic issue within the bone marrow. This condition is typically caused by a mutation in hematopoietic stem cells, leading to an overproduction of red blood cells regardless of the body's oxygen needs. This means that the increase in RBCs is fundamentally due to a disease process, particularly related to a myeloproliferative disorder—hence, the correct answer points to an increase in RBCs due to a disease.

In contrast, other options discuss variations such as increases in RBCs from external factors or normal production, neither of which captures the essence of primary erythrocytosis. The concept of decreased RBCs due to disease is entirely unrelated to primary erythrocytosis, focusing instead on conditions causing anemia rather than erythrocytosis. Thus, recognizing that primary erythrocytosis is specifically tied to a pathological condition within the bone marrow is essential for a clear understanding of the disorder.