A 38-year-old woman presents with progressive loss of pain and temperature sensation in a cape-like distribution over her shoulders and upper arms, while light touch and vibration sensation remain intact. She reports painless burns on her hands from touching a hot stove. MRI of the brain and spine reveals a longitudinal CSF-signal cavity within the spinal cord at the cervical level, with cerebellar tonsillar descent of 7 mm below the foramen magnum (shown as structure **A** in the diagram). Which of the following best explains the dissociated sensory loss pattern observed in this patient?