A 34-year-old woman from Bangalore presents with progressive weakness of both lower limbs over 3 weeks, with preserved pain and temperature sensation but loss of vibration sense and proprioception in the feet bilaterally. Examination shows hyperreflexia, positive Romberg sign, and a sensory level at T8. MRI shows a T2-hyperintense lesion in the dorsal columns spanning T6–T10. What is the most appropriate next step in management?