An 18-year-old female presents with visual acuity decline for 2 years and recurrent episodes of limb paralysis for 1 year, partially relieved after two hospitalizations. Findings include positive oligoclonal bands in cerebrospinal fluid (CSF) and abnormal visual and somatosensory evoked potentials. MRI shows multiple T2-weighted hyperintense lesions in cerebral white matter. The most likely diagnosis is: