Iron Deficiency Anemia: This is the most common cause of low MCV (mean corpuscular volume) and MCH (mean corpuscular hemoglobin) with a high RDW (red cell distribution width). The high RDW indicates a variation in red blood cell size, which is typical in iron deficiency anemia due to the gradual depletion of iron stores affecting hemoglobin synthesis and leading to smaller (microcytic) and more varied (anisocytic) red blood cells.

Thalassemia: These are genetic disorders affecting hemoglobin synthesis, leading to microcytic anemia. While they can present with a high RDW, especially in cases of thalassemia trait or when there's co-existing iron deficiency, the RDW can sometimes be normal or only slightly elevated. The key to diagnosing thalassemia often lies in the combination of microcytosis, a significant reduction in MCH, and specific hematologic indices.
Anisocytic Anemia: This term refers to anemia characterized by a significant variation in red blood cell size (anisocytosis), which would be reflected by a high RDW. While not a specific disease entity, it can be seen in various conditions, including mixed anemia where there's a combination of iron deficiency and another microcytic or normocytic anemia.
Chronic Disease: Chronic diseases, such as chronic kidney disease or chronic infections, can lead to a microcytic anemia with a high RDW due to the body's inflammatory response and possible co-existing iron deficiency or other nutritional deficiencies.

Sideroblastic Anemia: This is a group of disorders characterized by the presence of ringed sideroblasts in the bone marrow, indicating a defect in heme synthesis. It can present with microcytic anemia, high RDW, and often increased iron stores. Missing this diagnosis could lead to inappropriate treatment and failure to address the underlying cause.
Myelodysplastic Syndromes (MDS): These are clonal stem cell disorders that can present with various cytopenias, including anemia. A high RDW can be an early indicator of MDS, and missing this diagnosis could delay appropriate management and potentially life-saving interventions like bone marrow transplantation.

Congenital Disorders of Glycosylation: These are rare genetic disorders that can affect various aspects of cellular function, including glycosylation of proteins. Some types can present with microcytic anemia and a high RDW, among other systemic manifestations.
Erythrocyte Membrane Disorders: Disorders like hereditary spherocytosis or hereditary elliptocytosis can sometimes present with microcytic indices and a high RDW, especially if there's significant hemolysis and reticulocytosis leading to anisocytosis.