a 14-year-old treated for sinusitis 2 weeks ago presents to your ed with worsening headache and fevers over the last week. his parent states that he has been sleeping most of the day and brought him in because he was having trouble walking. he has had no vomiting, vision changes, photophobia, neck pain, or trauma. his vital signs are within normal limits for his age. your examination shows a pale-appearing, somnolent male who wakes and answers questions appropriately. he has a normal cranial nerve exam and negative kernig and brudzinski signs but is unable to heel-to-toe walk and has a foot drop on the left. of the following, what is the best next step to establish the diagnosis?