A 58-year-old Indian male presents with progressive difficulty walking, positive Romberg sign, and loss of vibration sense in both lower limbs. Serum B12 level is 180 pg/mL (normal >200). MRI spine shows T2 hyperintensity in the dorsal midline of the cervical and thoracic cord. The structure marked **B** in the diagram is primarily affected in this condition. Which of the following best explains the clinical presentation?