Subacute Combined Degeneration

A 68-year-old vegetarian woman presents with a 4-month history of progressive symmetric leg weakness, gait ataxia, and paresthesias in stocking distribution. Examination reveals loss of vibration and proprioception in the feet, brisk knee reflexes but absent ankle reflexes, extensor plantar responses, and a positive Romberg sign. Laboratory findings show hemoglobin 9.4 g/dL, MCV 112 fL, hypersegmented neutrophils, serum B12 90 pg/mL, elevated methylmalonic acid and homocysteine, and positive intrinsic factor antibodies. MRI of the cervical spine shows symmetric T2 hyperintensity in the posterior columns with an inverted V sign on axial images. The pathophysiology marked as **A** in the diagram involves impaired myelin synthesis due to depletion of which critical methyl donor?