A diaphragmatic apex below the 6th rib is consistent with

• A. Pleural effusion
• B. Under aeration
• C. Pneumothorax
• D. Diaphragmatic hernia

Answer: (C)

On a supine neonatal chest radiograph, a diaphragmatic apex that sits lower than expected—below the 6th rib—points to the presence of air in the pleural space, i.e., a pneumothorax. This appearance is known as the deep sulcus sign: when the patient is lying on their back, free air tends to accumulate anteriorly and inferiorly, which makes the diaphragmatic contour appear abnormally low and the lung markings at the periphery sparse or absent. This pattern helps distinguish pneumothorax from other issues.

Pleural effusion would more likely blunt or obscure the costophrenic angles and show fluid layering rather than a depressed diaphragmatic apex. Under aeration or atelectasis generally causes diffuse increased density and crowding of vessels rather than a specifically lowered diaphragmatic contour. A diaphragmatic hernia would present with abdominal contents or gas-filled loops in the chest and mediastinal shifts, not a low diaphragmatic apex alone.