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, bubble-gum tasting antibiotic." His mother reports that for the past 12 hours or so he has been holding his head saying it hurts and he is less active than normal. On your examination, he seems to have a severe headache, nystagmus, and ataxia. Which of the following would be the most appropriate first test to order?

The patient in the question has a brain abscess, a condition more commonly seen in patients with cardiac defects that have right-to-left shunts associated with them. The antibiotics that he received from the ER kept the condition somewhat under control, but was unlikely to cure the condition, which ultimately progressed. The diagnostic tool of choice is imaging, and either CT or MRI is indicated. Lumbar puncture would be contraindicated in this patient until after imaging (the patient is at risk for brain herniation) and the CSF (and blood) cultures are usually negative. The patient is not experiencing new cardiac problems, so an echocardiogram is not indicated immediately. If the patient has negative imaging of the brain, then a urine drug screen might be indicated, but not as a test of first choice.