Mr Smith is a 79 year old man who was recently diagnosed with prostate cancer. He is awaiting staging scans and then to be discussed at the urology cancer MDT meeting to explore treatment options. His next out-patient appointment is next week. Mr Smith presents to his GP surgery with lower back pain which radiates down his legs and that is worse with coughing. In addition, since arriving in the surgery, his legs feel ‘heavy’. He denies any bladder/bowel changes. Examination reveals tenderness over L1, reduced power in his lower limbs with increased tone and brisk reflexes. There is no obvious sensory level but anal tone is intact/perianal sensation is normal.
Which one of the below investigations would be most appropriate to confirm the likely diagnosis?