**Core Concept**
An air bronchogram is a radiographic sign seen on chest X-rays, where the normally invisible air-filled bronchi become visible due to the alveoli being filled with fluid or other materials. This occurs due to the contrast between the air-filled bronchi and the surrounding opacified alveoli, making the bronchi stand out.

**Why the Correct Answer is Right**
The air bronchogram sign is typically seen in conditions where there is alveolar consolidation, such as in pneumonia. When the alveoli are filled with fluid or exudate, the air in the bronchi becomes more visible due to the contrast. This is because the air in the bronchi is now surrounded by the opaque alveoli, making it stand out on the X-ray. The most common cause of this sign is lobar pneumonia, where the alveoli are filled with pus and fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** Pneumothorax - This condition would not typically produce an air bronchogram, as the air in the thoracic cavity is outside the lung parenchyma and would not be visible in the alveoli.

**Option B:** Pleural effusion - While a pleural effusion can cause an opacified area on the chest X-ray, it would not typically produce an air bronchogram, as the fluid is in the pleural space and not within the alveoli.

**Option C:** Pulmonary edema - While pulmonary edema can cause an opacified area on the chest X-ray, it would not typically produce an air bronchogram, as the fluid is in the interstitial spaces and not within the alveoli.

**Clinical Pearl / High-Yield Fact**
The air bronchogram sign is a classic radiographic finding in lobar pneumonia, and it is often used to distinguish this condition from other causes of opacification on the chest X-ray. Remember that an air bronchogram is a sign of alveolar consolidation, and it is typically seen in conditions where the alveoli are filled with fluid or exudate.

**Correct Answer:** C. Pulmonary edema