Q. A 63-year-old man presents to the emergency department with an abrupt onset of shortness of breath, cough, hypoxemia, and tachycardia. He has a history of gastroesophageal reflux disease and smokes a pack of cigarettes per day for the past 30 years. He occasionally drinks alcohol. He has a fever of 100.2°F (37.9°C), blood pressure of 135/85 mmHg, heart rate of 110 beats/min, respiratory rate of 28 breaths/min, and oxygen saturation of 88% on room air. Lung auscultation reveals diffuse crackles and wheezes. Chest imaging shows opacities involving the lower lobes. Which of the following is the most likely diagnosis?