A 48-year-old woman presents to your clinic with 3 weeks of worsening leg heaviness and difficulty with balance. She denies any bladder or bowel issues. On examination, she has 3/5 strength in the bilateral iliopsoas and quadriceps. Sagittal and axial T2 MRI sequences are shown in Figures A and B, respectively. Cervical MRI is unremarkable. What is the next most appropriate step in management?