A 2-month-old girl has had cough and congestion for several days, and her parents note increased respiratory difficulty over the past 2 days. Physical examination reveals nasal flaring, intercostal retractions, and decreased breath sounds on the right side of the chest. Afebrile and well-appearing, the infant receives 0.3 mL of normal saline via nebulizer, but no improvement is noted. Which of the following is the most appropriate next step?
a) Intravenous antibiotics
b) Chest X-ray
c) Admission for intravenous antibiotics and hydration
d) Continuous positive airway pressure (CPAP) therapy