Final answer:
Myelophthisic process is the pathologic event that impairs hematopoiesis by replacing the bone marrow, thus causing pancytopenia.
Step-by-step explanation:
The pathologic process that replaces bone marrow and impairs hematopoiesis, resulting in pancytopenia, is the Myelophthisic process. This process can involve various disease conditions where the bone marrow is replaced by abnormal or cancerous cells, leading to a decrease in the production of red blood cells (RBCs), white blood cells (WBCs), and platelets. When myeloid stem cells are defective or displaced by such cells, there won't be enough formed elements produced, which is crucial since these cells are continually being replaced due to their relatively short lifespan. In conditions like aplastic anemia or myelofibrosis, hematopoiesis is impaired, which further leads to deficits in RBC stem cells, contributing to anemia.