Which sign on a chest x-ray is specifically associated with pneumonia?

The presence of air bronchograms on a chest x-ray is a significant indicator associated with pneumonia. Air bronchograms occur when the air-filled bronchi are made visible by surrounding alveolar consolidation, which is characteristic of pneumonia. In a healthy lung, the bronchi are not typically visible on x-rays because they are surrounded by air-filled alveoli, which also contain air. When pneumonia develops, the alveoli become filled with fluid or inflammatory exudate, disrupting the normal air-filled architecture and allowing the outlines of the bronchi to be visualized against the opacified lung tissue.

This sign is particularly important for diagnosing certain types of pneumonia, especially in the presence of lobar pneumonia, where lung consolidation leads to clear outlines of the bronchi against denser consolidating tissue. It provides vital clues to clinicians about the underlying pathology and helps differentiate pneumonia from other respiratory conditions that might present similarly on imaging.