Final answer:
Aplastic anemia is mainly characterized by a decreased red blood cell count, due to a deficiency in RBC stem cells within the bone marrow, which can be congenital or caused by several external factors.
Step-by-step explanation:
The main feature of aplastic anemia is a decreased red blood cell count. This form of anemia is characterized by a deficiency in the production of red blood cells due to a lack of RBC stem cells in the bone marrow. The condition can be congenital or caused by external factors such as radiation, medication, chemotherapy, or infection. A decrease in reticulocyte count is an indicator of reduced RBC production, while increased levels of erythropoietin signal the body's attempt to compensate for the anemic state by trying to stimulate the bone marrow to produce more red blood cells. However, in aplastic anemia, the marrow is unable to respond appropriately, leading to persistently low RBC counts.