Neuromyelitis Optica Spinal Cord

A 34-year-old woman of Asian ancestry presents with 5 days of ascending bilateral leg weakness, band-like mid-thoracic pain with a sensory level at T6, urinary retention, and constipation. She had painful bilateral optic neuritis 2 months earlier. MRI of the whole spine with gadolinium shows longitudinally extensive transverse myelitis with central cord T2 hyperintensity. The spinal lesion marked **B** in the diagram—spanning greater than three vertebral bodies—is the defining characteristic that distinguishes this patient's condition from multiple sclerosis. Which of the following is the most appropriate long-term immunosuppressive agent for this patient's neuromyelitis optica spectrum disorder (NMOSD)?