**Core Concept**
The child's presentation is suggestive of a congenital anomaly involving the respiratory system, specifically affecting the left hemithorax. This anomaly leads to air-filled structures, causing mediastinal shift due to increased intrathoracic pressure.

**Why the Correct Answer is Right**
The presence of multiple air-filled structures in the left hemithorax, along with mediastinal shift, is characteristic of congenital diaphragmatic hernia (CDH). CDH occurs due to a defect in the diaphragm, allowing abdominal contents, including bowel loops and other organs, to herniate into the thoracic cavity. This results in compression of lung tissue and mediastinal shift. The air-filled structures seen on X-ray are likely bowel loops or stomach, which have herniated into the thoracic cavity.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pneumothorax typically presents with unilateral lung collapse, not multiple air-filled structures and mediastinal shift.
* **Option B:** Pleural effusion would not cause multiple air-filled structures in the hemithorax and mediastinal shift.
* **Option C:** Pneumonia may cause consolidation and increased lung opacity, not air-filled structures and mediastinal shift.

**Clinical Pearl / High-Yield Fact**
Congenital diaphragmatic hernia is a life-threatening condition that requires prompt diagnosis and management. It is essential to recognize the characteristic radiographic findings and clinical presentation to initiate timely treatment.

**Correct Answer:** C. Congenital diaphragmatic hernia.