**Core Concept**
Xerosis refers to a dryness of the skin or mucous membranes, often associated with various nutritional deficiencies, autoimmune disorders, or environmental factors. In ophthalmology, xerosis can manifest as dryness of the conjunctiva or cornea, potentially leading to complications such as keratoconjunctivitis sicca.

**Why the Correct Answer is Right**
Vitamin A deficiency is a well-established cause of xerosis, particularly in the context of night blindness, corneal ulcers, and dryness of the conjunctiva. This is because vitamin A plays a crucial role in the maintenance of the integrity and function of the cornea and conjunctiva, as well as in the production of mucins and lipids that help to lubricate and protect these tissues. The lack of vitamin A can lead to a breakdown in the integrity of these tissues, resulting in xerosis.

**Why Each Wrong Option is Incorrect**
**Option B:** Vitamin B deficiency is not directly associated with xerosis, although it can contribute to other ocular manifestations such as retinal degeneration and optic neuropathy.
**Option C:** Vitamin C deficiency, or scurvy, primarily affects collagen synthesis and can lead to symptoms such as poor wound healing, bleeding gums, and joint pain, but it is not typically associated with xerosis.
**Option D:** Vitamin D deficiency can lead to rickets and osteomalacia, but it is not directly linked to xerosis.

**Clinical Pearl / High-Yield Fact**
A classic exam question trick is to remember that vitamin A deficiency is associated with night blindness, while vitamin B deficiency is linked to optic neuropathy. This can help you distinguish between the correct and incorrect options.

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