**Core Concept**
Vitamin A deficiency is a public health concern worldwide, leading to various ocular and non-ocular manifestations. The condition arises due to inadequate dietary intake or impaired absorption of retinol, the active form of Vitamin A. This deficiency impairs the synthesis of rhodopsin, a critical protein in the retina responsible for low-light vision.

**Why the Correct Answer is Right**
Vitamin A plays a crucial role in maintaining the integrity and function of epithelial tissues, including the cornea, conjunctiva, and retina. In the retina, retinol is converted into 11-cis retinal, which binds to opsins to form rhodopsin. This complex is essential for phototransduction, the process of converting light into electrical signals in photoreceptor cells. Without adequate Vitamin A, these cells degenerate, leading to night blindness, a hallmark symptom of Vitamin A deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Vitamin A deficiency primarily affects the retina and cornea, rather than the lens or ciliary body.
**Option B:** This option is incorrect because Vitamin A deficiency is associated with impaired immune function, but it is not the primary characteristic of the condition.
**Option C:** This option is incorrect because Vitamin A deficiency can cause xerophthalmia, but it is not the most characteristic feature of the condition.

**Clinical Pearl / High-Yield Fact**
Vitamin A deficiency can be prevented through dietary supplementation, particularly in regions where Vitamin A-rich foods are scarce. The World Health Organization recommends administering high-dose Vitamin A to children with measles to prevent Vitamin A deficiency-related complications.

**Correct Answer:** D.