Brachial Plexus Traction Injury

A 26-year-old male motorcyclist presents with a complete right brachial plexus injury following a high-speed collision with lateral shoulder trauma and forced head deviation to the left. Examination reveals a flail right upper limb, complete sensory loss across C5–T1 dermatomes, absent all upper limb reflexes, right-sided Horner syndrome (ptosis, miosis, anhydrosis), and positive Tinel sign at the supraclavicular fossa. MRI of the cervical spine and brachial plexus obtained at 3 weeks post-injury shows the findings marked **A** protruding through the C7, C8, and T1 neural foramina with loss of expected rootlets. Which of the following best characterizes the pathophysiology indicated by the structure marked **A**?