**Core Concept:** The question is testing the ability to diagnose a respiratory condition in infants based on clinical findings and chest X-ray results. It involves understanding the pathophysiology of hyperinflation, clinical signs of respiratory distress, and correlating chest X-ray findings with the diagnosis.

**Why the Correct Answer is Right:**

The correct answer is **C. Bronchiolitis**. Bronchiolitis is a viral lower respiratory tract infection primarily affecting infants under one year of age. The clinical presentation includes respiratory distress, prolonged expiration with rhonchi, and hyperinflation on chest X-ray. The virus most commonly involved is respiratory syncytial virus (RSV). The child may also present with cough, feeding difficulties, and hypoxia. The chest X-ray findings of hyperinflation are indicative of reduced lung compliance due to viral infection and subsequent bronchospasm and inflammation of the small airways (bronchioles), leading to impaired air exchange and respiratory distress.

**Why Each Wrong Option is Incorrect:**

A. **Hyperinflation (hyperinflation is a normal finding in infants)**: This option is incorrect because the presence of hyperinflation is not consistent with a normal infant respiratory physiology. Infants are expected to have relatively smaller lungs, leading to increased lung compliance and decreased hyperinflation.

B. **Emphysema (emphysema is a chronic lung disease)**: This option is incorrect because emphysema is a chronic disease characterized by destruction of lung parenchyma, causing irreversible hyperinflation and reduced lung compliance. Emphysema is not relevant in an infant presenting with respiratory distress, prolonged expiration, and hyperinflation.

D. **Asthma (asthma is a chronic respiratory condition)**: This option is incorrect because asthma is a chronic respiratory condition affecting children and adults, not infants. The clinical presentation and chest X-ray findings in asthma differ from those in bronchiolitis.

C. **Bronchiolitis (bronchiolitis is a viral respiratory illness)**: This option is the correct answer. Bronchiolitis is a viral respiratory illness affecting infants, which presents with respiratory distress, prolonged expiration, and hyperinflation on chest X-ray. Bronchiolitis is primarily caused by RSV infection and presents with typical clinical manifestations and chest X-ray findings.

**Clinical Pearl:** Bronchiolitis is a common cause of respiratory distress in infants and can lead to severe respiratory distress syndrome (RDS) if not managed properly. Early recognition and management are crucial for preventing complications and improving the prognosis of these infants.