Macrocytic anemia with low folate and normal VB12; which presumptive diagnosis fits this pattern?

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Macrocytic anemia with low folate and normal B12 points to folate deficiency causing megaloblastic anemia. Folate is essential for thymidylate synthesis and DNA replication; when folate is lacking, DNA synthesis slows, especially in rapidly dividing marrow cells, leading to ineffective erythropoiesis and large, immature red cells (megaloblasts) that produce macrocytosis on the CBC. The fact that B12 is normal argues against B12 deficiency, which can also cause megaloblastic changes but would show low B12 levels (and often elevated methylmalonic acid). Sideroblastic and aplastic anemias do not present with this pattern of macrocytosis due to folate shortage alone—their marrow and blood smear findings differ (ring sideroblasts for sideroblastic, hypocellular marrow with pancytopenia for aplastic). So, this pattern best fits folate deficiency causing megaloblastic anemia.