A college student is examined at the emergency department; he is disoriented with a fever, intense headache, stiff neck, vomiting, and sensitivity to light. His friends say that he has been sick for about 2 days and that his condition worsened over the last 3 hours. The serum electrolytes are normal, but the patient's white blood count is 12,000 cells/L. What test should the physician order next?