## Core Concept
Air bronchogram is a radiologic sign that describes the appearance of air-filled bronchi made visible by the opacification of surrounding alveoli. This sign is typically associated with conditions that cause alveolar filling or consolidation.

## Why the Correct Answer is Right
The correct answer, **D. Pulmonary embolism**, is right because air bronchogram is not a characteristic feature of pulmonary embolism. Pulmonary embolism usually presents with nonspecific findings on a chest X-ray, such as the Westermark sign or Hampton's hump, but not air bronchogram. Air bronchograms are seen in conditions where there is alveolar consolidation, such as pneumonia.

## Why Each Wrong Option is Incorrect
* **Option A: Lobar pneumonia** - This condition often presents with air bronchograms due to the consolidation of the lung lobes. The alveoli are filled with inflammatory exudate, making the air-filled bronchi visible against the opaque lung tissue.
* **Option B: Acute respiratory distress syndrome (ARDS)** - ARDS is characterized by diffuse alveolar damage, leading to consolidation and potential air bronchograms on chest X-ray. This condition results from various insults, including sepsis, trauma, and pneumonia.
* **Option C: Pulmonary edema** - In severe cases, pulmonary edema can cause alveolar consolidation, which might reveal air bronchograms on a chest X-ray. However, this is less common than in conditions with more direct alveolar filling like pneumonia.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that air bronchograms are most commonly associated with alveolar consolidation, as seen in pneumonia. The presence of an air bronchogram suggests that the lung pathology is likely within the alveoli rather than in the airways or pulmonary vasculature.

## Correct Answer: D. Pulmonary embolism.