You are answering a USMLE-style medical question. Question: A 45-year-old woman comes to the physician because of a 2-week history of painful ulcers in her mouth. Over the past 2 months, she has had increasing fatigue and difficulties concentrating. She has a history of rheumatoid arthritis and was started on a new medication 4 months ago. Examination shows pallor of the mucosal membranes and three tender ulcerative lesions in her mouth. Her hemoglobin concentration is 8.7 g/dL and mean corpuscular volume is 109 μm3. A blood smear shows hypersegmented neutrophils. Which of the following is the most likely cause of this patient's findings? Options: A) Deficient heme synthesis B) Deficient erythropoietin secretion C) Deficient nitrogenous base production D) Deficient β-globin chains Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.