**Core Concept**
Periodic Acid Schiff (PAS) stain is a histopathological technique used to detect glycogen, glycoproteins, and polysaccharides in tissue sections. It is particularly useful in identifying intra-alveolar material, which is often a diagnostic challenge in pulmonary pathology.

**Why the Correct Answer is Right**
The PAS stain detects the presence of glycogen and glycoproteins in the intra-alveolar material. In conditions like alveolar proteinosis, the intra-alveolar material is rich in surfactant protein and phospholipids, which are PAS-positive. This is because the surfactant protein and phospholipids contain carbohydrate moieties that are stained by the PAS reagent. The PAS stain is a key diagnostic tool in differentiating alveolar proteinosis from other conditions like pulmonary edema or alveolar hemorrhage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a condition that is associated with PAS-positive intra-alveolar material. While some conditions like pulmonary edema may show intra-alveolar material, it is not typically PAS-positive.
**Option B:** This option is incorrect because it is a vague term that does not specify a particular condition. Intra-alveolar material can be seen in many conditions, but not all of them are PAS-positive.
**Option C:** This option is incorrect because it is a condition that is associated with intra-alveolar material, but it is not typically PAS-positive. Pulmonary edema is a condition where fluid accumulates in the alveoli, but it is not characterized by PAS-positive material.

**Clinical Pearl / High-Yield Fact**
The PAS stain is a valuable tool in identifying glycogen and glycoproteins in tissue sections. It is particularly useful in diagnosing conditions like alveolar proteinosis, where the intra-alveolar material is rich in surfactant protein and phospholipids.

**Correct Answer: C. Pulmonary edema is associated with intra-alveolar material but is not typically PAS-positive.