A 28-year-old woman presents with a 2-year history of occipital headaches worsened by coughing and Valsalva maneuver. She reports downbeat nystagmus, palatal weakness, and cape-like sensory loss over the shoulders. Sagittal T1-weighted MRI of the craniocervical junction shows the structure marked **A** extending 9 mm below the foramen magnum with effacement of posterior fossa CSF spaces and a small cervical syrinx. The fourth ventricle remains in normal position and the brainstem is not displaced. Which of the following best defines the pathophysiology of this finding?
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