**Core Concept**
Corneal opacity refers to the clouding or loss of transparency of the cornea, which is the clear, dome-shaped surface at the front of the eye. This condition can result from various causes, including injury, infection, or degenerative changes. Corneal opacity can lead to vision impairment and may require surgical intervention to restore vision.

**Why the Correct Answer is Right**
Corneal opacity can be caused by the deposition of abnormal proteins or lipids within the corneal stroma, leading to a loss of transparency. This can occur due to conditions such as keratoconus, Fuchs endothelial dystrophy, or after a corneal injury. The cornea's unique structure, with its tightly packed collagen fibers and absence of blood vessels, makes it susceptible to opacity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because corneal opacity can indeed be caused by an increase in corneal thickness, such as in cases of keratoconus or edema. However, this is not the only cause of corneal opacity.
**Option B:** This option is incorrect because corneal opacity can be a result of the scarring process following an injury, but it is not the sole cause of opacity.
**Option C:** This option is incorrect because corneal opacity can be caused by a decrease in corneal hydration, such as in cases of Fuchs endothelial dystrophy, but this is not the only cause.

**Clinical Pearl / High-Yield Fact**
The cornea's transparency is maintained by a delicate balance of hydration and protein-lipid interactions within the corneal stroma. Disruptions to this balance can lead to corneal opacity, highlighting the importance of maintaining a healthy corneal environment.

**Correct Answer: A.**

✓ Correct Answer: C. A strictly localized dense leucoma interferes more than a thin,diffuse nebula covering the papillary area.