Pneumonia USMLE Step 1 Practice Question

A 28-year-old woman with no significant past medical history presents to the emergency department with a 5-day history of fever, nonproductive cough, and generalized malaise. She reports her symptoms began gradually after exposure to coworkers who were ill. She denies pleuritic chest pain, hemoptysis, or dyspnea on exertion. Vital signs include temperature 39.1°C, heart rate 105 bpm, respiratory rate 22/min, blood pressure 120/78 mmHg, and oxygen saturation 94% on room air. Physical examination reveals bilateral crackles on auscultation. Chest X-ray shows bilateral interstitial infiltrates without consolidation. Laboratory studies demonstrate a white blood cell count of 7,800/μL with normal differential. Sputum Gram stain shows sparse white blood cells and no organisms. Cold agglutinin test is positive at 1:64 titer. Which of the following is the most likely etiology of this patient's pneumonia?