Okay, let's tackle this question about the stain used in Granular dystrophy of the cornea. First, I need to recall what Granular dystrophy is. It's a type of corneal dystrophy, right? Specifically, it's one of the three main types—probably along with lattice and macular. The key here is the staining method used to visualize the deposits.

The options aren't given, but the correct answer is likely Congo red stain. Wait, why? Because Granular dystrophy is characterized by amyloid deposits. Amyloid stains with Congo red, which gives an apple-green birefringence under polarized light. That's a classic finding.

Let me verify. Corneal dystrophies are grouped into different types. Granular dystrophy (also called Avellino dystrophy) involves amyloid-like material accumulation. The deposits are granular and can be stained with Congo red. Lattice dystrophy might use PAS or Alcian blue. Macular dystrophy is more about lipid accumulation, so maybe Sudan black or Oil Red O.

So the core concept is that Granular dystrophy is an amyloidosis of the cornea, and Congo red is the stain used. The other options would be for different types. For example, if an option was PAS, that's for lattice. Alcian blue for lattice too. Oil Red O for macular. So each dystrophy has its own stain. The pearl here is remembering that Congo red is for amyloid, which is in Granular dystrophy. Also, the apple-green birefringence under polarized light is a key diagnostic feature. Got it.

**Core Concept**
Granular corneal dystrophy (Avellino dystrophy) is an autosomal dominant disorder characterized by amyloid-like protein deposits in the corneal stroma. The diagnosis relies on specific histopathological staining to identify these deposits, distinguishing it from other corneal dystrophies like lattice or macular.

**Why the Correct Answer is Right**
Congo red stain is the gold standard for detecting amyloid deposits in granular corneal dystrophy. Under polarized light, amyloid deposits exhibit characteristic apple-green birefringence. This occurs because Congo red binds to β-pleated sheet structures in amyloid proteins, a hallmark of this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** PAS (Periodic Acid-Schiff) is used for lattice corneal dystrophy, which shows amyloid with a lattice pattern but does not show birefringence.
**Option B:** Alcian blue stains mucopolysaccharides, relevant in macular corneal dystrophy (linked to carbohydrate deposits).
**Option C:** Oil Red O detects lipid inclusions, which are not a feature of granular dystrophy but seen in other corneal conditions like lipid keratopathy.

**Clinical Pearl / High-Yield Fact**
Remember **"Congo red = amyloid + apple-green"** for granular dystrophy. Always associate polarized light microscopy with amyloid confirmation. Lattice dystrophy uses PAS/Alcian blue, while macular dystrophy uses Oil Red O.

**Correct Answer: C. Congo red stain**

✓ Correct Answer: B. Masson's trichrome