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Brainstem PML lesion mimicking MS plaque in a natalizumab-treated MS patient

A 47-year-old woman with relapsing-remitting multiple sclerosis (MS), treated with natalizumab for 14 months, reported dysphagia. Brain MRI demonstrated small fluid-attenuated inversion recovery–hyperintense T1-hypointense unenhancing lesions of the right pons and left medulla initially interpreted as new MS plaques (figure 1), but increased and became confluent over 3 months (figure 2). CSF demonstrated the presence of JC virus (4,015 DNA copies).