**Core Concept**
The patient is likely suffering from a congenital diaphragmatic hernia (CDH), a condition where the diaphragm fails to develop properly, allowing abdominal organs to herniate into the chest cavity. This can lead to respiratory distress and compromise the infant's ability to breathe.

**Why the Correct Answer is Right**
The maximal cardiac impulse shifted further to the right side after endotracheal intubation, indicating that the herniated abdominal contents compressed the left lung and mediastinum further. The presence of a scaphoid abdomen and decreased breath sounds on the left side also supports this diagnosis. In CDH, the herniated contents can compress the left lung and mediastinum, leading to respiratory distress and a shift of the cardiac impulse to the right.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chest X-ray is not the immediate next step in management. While it can confirm the diagnosis of CDH, the patient's respiratory status requires immediate attention.
* **Option B:** Surgical repair is not the immediate next step in management. The patient requires stabilization and optimization of respiratory status before undergoing surgery.
* **Option D:** ECMO is not the immediate next step in management. While it may be considered in severe cases of respiratory failure, the patient's condition can be managed with endotracheal intubation and mechanical ventilation.

**Clinical Pearl / High-Yield Fact**
In CDH, the scaphoid abdomen is due to the herniated abdominal contents compressing the abdominal contents, leading to a scaphoid (boat-shaped) appearance.

**Correct Answer:** C. Surgical repair is the definitive treatment for CDH, but the patient requires stabilization and optimization of respiratory status before undergoing surgery. The next step in management would be to optimize the patient's respiratory status with mechanical ventilation and consider other supportive measures such as ECMO if necessary.