A 32-year-old woman with relapsing-remitting multiple sclerosis (MS) reports episodes of blurred vision and lower-extremity weakness that partially resolve over several months. After 12 years of disease, she develops steadily worsening gait impairment and bowel and bladder dysfunction, with minimal recovery between exacerbations. MRI reveals extensive chronic plaques in the spinal cord and periventricular white matter. Which pathophysiologic mechanism best explains the patient’s progression from relapsing symptoms to permanent neurologic disability?