翻译 A 58-year-old man with a basilar tip aneurysm is referred by a neurosurgeon. He has a 4-year history of progressive spastic paraparesis. He has recently had urge incontinence of urine. He also has numbness in the right toes more than the left, and pain in the thighs and back. There have been some gradual fluctuations, but no clear, discrete episodes of deterioration. He has had no disturbances of vision, eye movement, or motor control of the upper extremities. He was referred when surgical clipping of the aneurysm 3 month ago failed to help his symptoms. Which of the following is the most appropriate next diagnostic test?