**Core Concept**
Corneal vascularization is a condition characterized by the growth of blood vessels into the cornea, leading to vision impairment. This phenomenon is often associated with nutritional deficiencies, particularly in vitamins that play a crucial role in maintaining corneal health.

**Why the Correct Answer is Right**
The correct answer is Vitamin A. Vitamin A is essential for maintaining the integrity of the corneal epithelium and preventing corneal vascularization. A deficiency in Vitamin A leads to impaired corneal epithelialization, allowing blood vessels to penetrate the cornea. This process is mediated by the reduction of the corneal epithelial basement membrane, which normally acts as a barrier to vascular growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin B12 is not directly associated with corneal vascularization. While Vitamin B12 deficiency can cause various systemic symptoms, it is not specifically linked to corneal vascularization.
**Option B:** Vitamin C is important for collagen synthesis and maintaining connective tissue health, but it is not directly implicated in corneal vascularization.
**Option C:** Vitamin D is crucial for bone health and calcium metabolism, but it does not play a primary role in preventing corneal vascularization.

**Clinical Pearl / High-Yield Fact**
Corneal vascularization is a sign of nutritional deficiency, particularly in Vitamin A, and can be a precursor to more severe ocular complications. It is essential to recognize and address nutritional deficiencies promptly to prevent vision impairment.

**Correct Answer: D. Vitamin A deficiency.**