(a) Source: (Hay et al, pp 512-513. Kliegman et al, pp 1456-1459. McMillan et al, pp 699-700, 1391-1394. Rudolph et al, pp 962, 963, 966.) Of the choices given and with the findings on the radiograph (patchy infiltrates with flat diaphragms), monitoring oxygenation and hydration status is the most appropriate course of action as bronchiolitis is the most likely diagnosis. Bronchodilators and a short course of steroids are treatments for asthma, a less likely diagnosis in this patient without previous wheezing episodes and without a family history of atopy. A single dose of short-acting b-agonist might be tried in this patient but would be expected to be of limited benefit. A single dose of steroids would be an appropriate treatment for viral croup; intubation and antibiotics would be the approach to a patient with epiglottitis. Chest tube placement for a pneumothorax may be required if unilateral breath sounds were absent or if the radiograph demonstrated collapse; neither is noted in this case. The most likely cause of the illness is infection by respiratory syncytial virus, which causes outbreaks of bronchiolitis of varying severity, usually in the winter and spring. Other viruses, such as parainfluenza and the adeno-viruses, have also been implicated in producing bronchiolitis. Treatment is generally supportive in this usually self-limited condition.