What is the most characteristic change seen in the WBCs of the megaloblastic anemia bone marrow?

The most characteristic change seen in the white blood cells of bone marrow in megaloblastic anemia is the presence of giant metamyelocytes. In megaloblastic anemia, there is impaired DNA synthesis due to deficiencies in vitamin B12 or folic acid, leading to abnormal development of hematopoietic cells. As a result, the cells undergo abnormal maturation and enlargement, specifically affecting the myeloid series.

Giant metamyelocytes are indicative of this abnormal maturation process, as they are significantly larger than normal metamyelocytes and may display other atypical morphological features. This enlargement is a direct consequence of the ineffective erythropoiesis and disrupted myeloid differentiation that characterize megaloblastic processes.

This distinct morphological change helps in diagnosing megaloblastic anemia during microscopy of a bone marrow aspirate, making it a key feature of the condition.