An 18-year-old female presents with visual acuity decline for 2 years and recurrent episodes of limb paralysis for 1 year, with partial relief after two hospitalizations. Positive oligoclonal bands in cerebrospinal fluid (CSF) and abnormal visual and somatosensory evoked potentials were noted. MRI shows multiple T<sub>2</sub>-weighted hyperintense lesions in white matter. The preliminary diagnostic basis for this patient is: