Review Question - QID 215045

A 23-year-old woman presents to the emergency room due to persistent dyspnea over the past 2 days. She states that she has had 3 episodes of pneumonia over the last 2 years, and 2 episodes of infectious diarrhea with 1 episode of bloody diarrhea requiring intensive care 9 months ago. She works as a consultant and has not traveled outside of the United States. She does not smoke cigarettes, drink alcohol, or use any illicit drugs. She is currently on an oral contraceptive pill. She eats a healthy diet and exercises 3 times a day, and all her immunizations are up to date. Her temperature is 100.5°F (38.1°C), blood pressure is 128/80 mmHg, pulse is 84/min, and respirations are 22/min. On physical exam, she has notable crackles over her left lower lung field. There are no other abnormalities. Human immunodeficiency virus antibody testing is negative. Her leukocyte count is 16,000/mm^3. Chest radiography reveals a left lower lobe infiltrate. Which of the following is the most appropriate next step in management?