A 30-year-old woman with leukemia presents to the emergency room with fever for less than 24 hours. A week previously, she completed a course of chemotherapy through a Hickman catheter. She only complains of fever and fatigue. On examination, she is pale and febrile (101.5° F). Laboratory evaluation reveals a leukocyte count of 780 with 25% polymorphonuclear cells. Which of the following is the most appropriate therapy?