A 62-year-old woman presents to accident and emergency with a 1-day history of sudden onset back pain and difficulty walking. She has not opened her bowels or passed urine for the previous day. She has a past medical history of breast cancer, diagnosed two years earlier and staged as T2N1M0 disease with oestrogen receptor positive status. She has been treated for her cancer with a wide local excision and axillary node clearance, followed by radiotherapy, chemotherapy and tamoxifen. On examination, there is reduced tone in the lower limbs. Power is diminished throughout the lower limbs, but especially on hip flexion. There is reduced sensation below the L1 dermatome. What is the most appropriate diagnostic investigation?