A 2-year-old boy is brought to the emergency department with a 3-day history of persistent cough and recurrent left-sided wheezing. The mother reports a choking episode while eating peanuts 3 days ago, but the child seemed to recover. Chest X-ray in expiration shows the finding marked **A** in the diagram. Which of the following best explains the pathophysiology of this radiological finding?

Explanation

## Why ball-valve obstruction is right The classic radiological finding in foreign body aspiration is unilateral hyperinflation on expiratory chest X-ray, exactly as marked **A** in the diagram. This occurs because the aspirated foreign body (in this case, peanut) acts as a one-way valve: it permits air entry during inspiration when intrathoracic pressure is negative, but obstructs air egress during expiration when intrathoracic pressure becomes positive. This air-trapping mechanism causes the affected lung to remain lucent and hyperinflated on expiration, while the mediastinum shifts away (as seen in **B**) and the ipsilateral diaphragm depresses (**C**). The 3-day delay and "silent period" after the initial choking episode is a classic diagnostic pitfall—reflexes adapt and the child may appear well between episodes. Nelson Pediatrics 22e emphasizes that expiratory CXR is the key imaging study because the ball-valve mechanism is most evident during expiration. ## Why each distractor is wrong - **Complete airway obstruction**: Complete obstruction would cause immediate post-obstructive atelectasis (collapse) with mediastinal shift TOWARD the affected side, not away. This child survived 3 days, ruling out complete obstruction. The hyperinflation pattern is opposite to atelectasis. - **Bronchospasm with bilateral hyperinflation**: While aspiration can trigger bronchitis and bronchospasm, this would produce bilateral findings, not the unilateral hyperinflation with contralateral mediastinal shift seen here. The asymmetry is pathognomonic for mechanical obstruction. - **Tension pneumothorax**: Tension pneumothorax would present acutely with severe respiratory distress, hypoxia, and hemodynamic collapse. The 3-day indolent course and the specific pattern of hyperinflation with depressed diaphragm are inconsistent with pneumothorax. **High-Yield:** Unilateral hyperlucent lung + mediastinal shift away + expiratory hyperinflation = ball-valve FB obstruction; rigid bronchoscopy is the gold standard for diagnosis and retrieval. [cite: Nelson Pediatrics 22e Ch 418]