Anemia characterized by the presence of unusually large erythroblasts in the bone marrow called megaloblasts. It is usually caused by vitamin B12 or folic acid deficiency. Other causes include toxins and drugs. The disease is megaloblastic anemia (MONDO_0001700). Also known as: Grasbeck-Imerslund syndrome, MGA1 Norwegian type, RH-MGA1, megaloblastic anaemia (disease), megaloblastic anemia (disease), recessive hereditary megaloblastic anaemia 1, recessive hereditary megaloblastic anemia 1.