A 23-year-old woman presents with arm weakness, decreasing vision in her right eye, and difficulty with her balance. An MRI of the brain reveals multiple demyelinating lesions.For the above patient with neurologic symptoms, select the most likely structural pathology.

Myelin is a complex protein lipid carbohydrate structure, which forms part of the cell membrane of the oligodendroglia. Vascular lesions cause demyelination because of ischemia. Papovaviruses can cause progressive multifocal leukoencephalopathy in patients with HIV infection, or less commonly, malignancy. Acute disseminated encephalomyelitis has been described after smallpox or rabies vaccination. Nutritional deficiencies can also cause demyelination (eg, pernicious anemia with vitamin B12 deficiency).