**Core Concept**
Air bronchogram is a radiological sign seen on chest X-rays, where normally invisible bronchial markings become visible due to the alveoli being filled with fluid or other materials, making them appear as if they are filled with air. This sign is particularly useful in identifying conditions that affect the lung parenchyma.

**Why the Correct Answer is Right**
The air bronchogram sign is typically seen in conditions that cause consolidation or infiltration of the lung parenchyma, such as pneumonia, pulmonary edema, or alveolar hemorrhage. In these conditions, the alveoli are filled with fluid, which makes the normally invisible bronchial markings visible due to the contrast between the air-filled bronchi and the fluid-filled alveoli. This occurs because the air-filled bronchi act as a "contrast agent" that highlights the surrounding fluid-filled alveoli.

**Why Each Wrong Option is Incorrect**
**Option A:** Air bronchogram is not typically seen in conditions that cause airway obstruction, such as bronchial asthma. However, it can sometimes be seen in severe cases of asthma where there is associated pulmonary edema or atelectasis.

**Option B:** Air bronchogram is not typically seen in conditions that cause air trapping, such as chronic obstructive pulmonary disease (COPD). However, it can sometimes be seen in severe cases of COPD where there is associated pulmonary edema or atelectasis.

**Option C:** Air bronchogram is not typically seen in conditions that cause pleural effusion. However, it can sometimes be seen in cases where the pleural effusion is large enough to cause significant alveolar collapse or atelectasis, which can lead to the formation of an air bronchogram.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the air bronchogram sign is a sign of alveolar consolidation or infiltration, and not just a sign of airway obstruction or pleural effusion. This can help you to narrow down the differential diagnosis based on the radiological findings.

**Correct Answer:** D.