An essential characteristic of megaloblastic anemia is an increased mean cell volume (MCV), in general greater than 100 fL. Most cases of megaloblastic anemia are a result of vitamin B12 (also known as cobalamin) and/or folic acid deficiency. Nuclear maturation does not occur at the normal rate, resulting in a nucleus that does not condense in size as the RBC develops. In peripheral blood, macrocytic and ovalocytic RBCs and hypersegmented neutrophils are classic. Howell-Jolly bodies are commonly seen in RBC's. Changes can also be identified in the bone marrow, especially abnormal nuclear maturation in erythroid precursors, giant metamyelocytes, and increased mitotic figures. It is of note that hypersegmented neutrophils in the peripheral blood may be an early sign of megaloblastic changes to come, including full megaloblastic anemia. This is because hematopoietic precursor cells in the bone marrow are some of the most quickly dividing cells in the body and are extremely sensitive to abnormal DNA synthesis caused by decreasing vitamin B12 and folate.
In children, vitamin B12 and folate deficiencies are often due to malnutrition. Malnutrition is not common in the United States, so conditions such as severe anorexia (as seen in this patient) or reduced food intake secondary to severe illness are often at fault.