Step 1: Acute bronchiolitis is overwhelmingly a viral illness. The commonest cause is respiratory syncytial virus (RSV), followed by parainfluenza, adenovirus and influenza viruses.
Step 2: Because the cause is viral, antibiotics such as macrolides have no role in routine treatment. Management is mainly supportive and includes a humid atmosphere with oxygen, nebulized bronchodilators (beta agonists, ipratropium, epinephrine) and antipyretics.
Step 3: When a specific antiviral is indicated, ribavirin is the agent of choice. Since macrolides are not part of standard care, the answer is macrolides.