Multiple Sclerosis — Dawson Fingers

A 28-year-old woman presents with subacute painful unilateral vision loss, reduced color perception, and a relative afferent pupillary defect. She reports a prior episode of right hemibody numbness 6 months ago. Examination reveals Lhermitte's sign and a sensory level at T6. MRI brain shows multiple ovoid T2/FLAIR hyperintense lesions in the periventricular white matter oriented perpendicular to the lateral ventricles, as marked **A** in the diagram. Gadolinium-enhanced imaging shows both active and chronic lesions. CSF analysis reveals oligoclonal bands and elevated IgG index. Which of the following best describes the pathological basis of the lesions marked **A**?