Q. A 56-year-old man presents to the emergency department with a 5-day history of fever, chills, cough, and shortness of breath. He also complains of nausea, vomiting, and diarrhea. His vital signs are as follows: temperature 38.9°C (102°F), heart rate 110 bpm, respiratory rate 22 breaths/min, blood pressure 130/85 mmHg, and oxygen saturation 92% on room air. His physical examination reveals crackles in the right lower lung field. Laboratory results show hyponatremia, elevated hepatic transaminases, and a C-reactive protein level of 120 mg/L. A chest X-ray reveals a right lower lobe infiltrate. The patient was initially treated with a beta-lactam antibiotic, but there has been no improvement in his condition. What is the most likely causative organism of the patient's pneumonia?