**Core Concept**
Bronchiolitis is an inflammatory condition of the small airways, primarily affecting infants and young children. It is most commonly caused by viral infections, particularly respiratory syncytial virus (RSV), which leads to airway obstruction and increased mucus production.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of bronchiolitis. The condition is characterized by an influx of inflammatory cells, including neutrophils and eosinophils, into the airways. This results in airway obstruction, increased mucus production, and impaired gas exchange. The RSV infection triggers a strong immune response, leading to the release of various cytokines and chemokines that exacerbate inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is true, as bronchiolitis is indeed a common cause of hospitalization in infants and young children. It is a leading cause of respiratory illness in this age group.

**Option B:** This statement is also true, as RSV is the most common cause of bronchiolitis. The virus is highly contagious and spreads through direct contact with infected individuals or contaminated surfaces.

**Option C:** This statement is not true. Bronchiolitis is not typically associated with an increase in airway diameter. In fact, the condition is characterized by airway obstruction, which can lead to wheezing and difficulty breathing.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of bronchiolitis is the presence of wheezing, which is often accompanied by a characteristic "seesaw" pattern of chest movement. This is due to the asynchronous movement of the diaphragm and the small airways, which can be observed on physical examination.

**Correct Answer:** C.