Q. A 2-year-old boy is brought to the pediatric clinic by his mother, who reports that her son has had a fever, runny nose, and cough for the past 3 days. The boy appears fatigued and has been less active than usual. His vital signs are: temperature 101.6°F (38.7°C), blood pressure 90/60 mmHg, heart rate 120 bpm, respiratory rate 30 breaths/min, and oxygen saturation 95% on room air. On auscultation, you notice wheezing and crackles in bilateral lung fields. The mother reports no recent contact with sick individuals, and the boy is up-to-date on vaccinations. What is the most common causative organism for pneumonia in this age group?