Which radiographic feature differentiates pneumothorax from severe atelectasis on chest radiographs?

• A. Atelectasis is defined by a visceral pleural line.
• B. Pneumothorax shows diffuse vascular crowding behind the heart.
• C. Pneumonia is indicated by a visceral pleural line.
• D. Pneumothorax shows a visceral pleural line with absent peripheral markings beyond it.

Answer: (D)

The defining feature is a visceral pleural line with absent peripheral markings beyond it, which signals air in the pleural space (pneumothorax). On chest radiographs, the edge of the collapsed lung appears as a sharp line (the visceral pleura) and beyond this line there are no markings because air separates the lung from the chest wall. Severe atelectasis, by contrast, shows volume loss with crowding of the interstitial and vascular markings and diaphragmatic or fissural shifts, but it does not create a distinct visceral pleural edge with a clear gap of non-ornamental markings beyond it. The other cues described in those options don’t isolate pneumothorax the way the visceral pleural line with absent peripheral markings does.