A newborn with cyanosis and chest X-ray showing left-sided cystic appearing areas and decreased aeration on the right. The most likely diagnosis is:

• A. Tracheoesophageal fistula
• B. Pyloric stenosis
• C. Congenital diaphragmatic hernia
• D. Meconium ileus

Answer: (C)

When a newborn is cyanotic and the chest X-ray shows left-sided cystic, air-filled areas in the thorax with the right lung showing decreased aeration, the most likely diagnosis is congenital diaphragmatic hernia. This condition arises when the pleuroperitoneal membranes fail to close properly, allowing abdominal contents to herniate into the chest. The herniated bowel in the left chest compresses the developing left lung, leading to pulmonary hypoplasia and overall respiratory distress, which explains the cyanosis.

This pattern is not typical for the other conditions. A tracheoesophageal fistula often presents with choking or respiratory symptoms and may show gas in the stomach or proximal bowel depending on the fistula, but the characteristic chest X-ray finding of bowel loops occupying the thorax is not the hallmark. Pyloric stenosis causes nonbilious projectile vomiting with a normal chest X-ray so it wouldn’t explain chest findings. Meconium ileus leads to intestinal obstruction signs with gas-filled loops in the abdomen rather than in the chest.

In short, left-sided herniation of abdominal contents into the chest producing bowel-like, cystic structures and a relatively under-aerated right lung fits congenital diaphragmatic hernia and its impact on neonatal respiration. This condition is a surgical emergency after stabilization.