A 34-year-old woman presents with a 6-month history of progressive occipital headaches triggered by coughing and Valsalva maneuver, along with bilateral upper extremity pain and temperature loss in a cape-like distribution with preserved proprioception. MRI brain shows that the structure marked **A** extends 7 mm below the foramen magnum. Whole-spine MRI reveals a cervical syringomyelia. Which of the following best describes the pathophysiological mechanism linking the structural abnormality marked **A** to the sensory deficit pattern observed?