Review Question - QID 214380

A 48-year-old man presents to his physician with sharp lower back and left lower extremity pain. He describes these painful episodes as sudden, severe, and lasting a few minutes. He denies any trauma to the lower back, lifting any heavy objects, but endorses episodes of urinary incontinence. He has fallen 3 times while walking in the dark and attributes it to "not knowing where my feet are before landing on the floor." Neurological examination is notable for his pupils not reacting to light but constricting to accommodation. His strength is normal, has diminished vibration and proprioception sensation, and has depressed ankle reflexes. He is unstable during Romberg testing. Which of the following is the most appropriate diagnostic study for this patient?