**Core Concept:** Bronchiolitis is a viral respiratory illness affecting infants, primarily due to respiratory syncytial virus (RSV) infection. It is characterized by inflammation and swelling of the small airways in the lungs, leading to respiratory distress.

**Why the Correct Answer is Right:** The correct answer, D, refers to the most common cause of bronchiolitis in infants. Respiratory syncytial virus (RSV) is a leading cause of bronchiolitis in young children, particularly those aged under 2 years. It is a common respiratory virus that can lead to severe respiratory distress in infants, often requiring hospitalization.

**Why Each Wrong Option is Incorrect:**

A. Respiratory infections due to bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae) are rare in bronchiolitis cases, as they primarily affect older children and adults.

B. While some viral infections like influenza or adenovirus can cause respiratory distress, they are less common than RSV in the context of bronchiolitis in infants.

C. Parainfluenza virus is another possible cause of bronchiolitis, but it is less commonly associated with severe respiratory distress in infants compared to RSV.

**Clinical Pearl / High-Yield Fact:** RSV infection is a major cause of bronchiolitis in infants, particularly in those under 2 years old. Clinicians must consider RSV testing when evaluating a young child with bronchiolitis, as early diagnosis and treatment can significantly improve outcomes. Prompt initiation of supportive care, such as oxygen supplementation and hydration, is crucial, along with monitoring for complications like pneumonia and bronchopneumonia.