A 38-year-old woman from rural Maharashtra presents with a 3-month history of progressive fatigue, dyspnea on exertion, and recurrent oral ulcers. On examination, she has mild hepatosplenomegaly and generalized lymphadenopathy. Laboratory investigations reveal: Hemoglobin 7.2 g/dL, WBC 2,100/μL, platelet count 65,000/μL, reticulocyte count 0.8%, and serum B12 level 180 pg/mL (normal >200). Peripheral blood smear shows hypersegmented neutrophils and macro-ovalocytes. Bone marrow examination shows hypercellular marrow with megaloblastic changes and increased iron stores. What is the most likely diagnosis?