A 2-year-old boy has been doing well despite his diagnosis of tetralogy of Fallot. He presented to an outside ER a few days ago with a complaint of an acute febrile illness for which he was started on a “pink antibiotic.” His mother reports that for the past 12 hours or so he has had a headache and is more lethargic than normal. On your examination he seems to have a severe headache, nystagmus眼球震颤, and ataxia运动失调. Which of the following would be the most appropriate first test to order?