A 58-year-old Indian woman presents with progressive fatigue and paresthesias in her feet for 6 months. On examination, the structure marked **A** in the diagram is noted — a beefy-red, smooth, atrophic appearance with loss of normal papillae. Laboratory investigations reveal hemoglobin 8.5 g/dL, MCV 128 fL, and serum B12 level of 150 pg/mL. Peripheral blood smear shows hypersegmented neutrophils. Which of the following is the most likely underlying mechanism responsible for the glossitis seen in this patient?