A 58-year-old man with a 15-year history of type 2 diabetes mellitus presents for neurological examination. When a vibrating tuning fork (128 Hz) is placed over his medial malleolus, he cannot perceive the vibration, though he can feel light touch and pain sensation normally. The structure marked **B** in the diagram is responsible for detecting this vibration stimulus. Which of the following pathological processes is most likely responsible for the loss of vibration sense in this patient?