**Core Concept**
Vitamin A deficiency can lead to impaired vision, particularly at night, and is associated with recurrent infections due to its role in maintaining the integrity of the mucosal surfaces and the immune system.

**Why the Correct Answer is Right**
Vitamin A plays a crucial role in the visual cycle, where it is converted into 11-cis retinal, which binds to rhodopsin in the photoreceptor cells of the retina. This binding is essential for the regeneration of rhodopsin, allowing the eye to adapt to low light levels. Additionally, vitamin A is essential for the maintenance of the epithelial surfaces, including the conjunctiva, where it helps to prevent the adhesion of pathogens and supports the production of mucins. The patient's symptoms, including diminished vision at night and dryness of the eyes, along with the presence of white triangular plaques on the conjunctiva, are consistent with a diagnosis of vitamin A deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Vitamin D deficiency is primarily associated with bone health and calcium metabolism, and is not directly linked to vision or recurrent infections.
**Option B:** Vitamin B deficiency can lead to a range of symptoms, including fatigue and neurological problems, but is not directly associated with impaired vision or recurrent infections.
**Option D:** Vitamin C deficiency can lead to scurvy, which is characterized by symptoms such as bleeding gums and joint pain, but is not directly associated with impaired vision or recurrent infections.

**Clinical Pearl / High-Yield Fact**
Vitamin A deficiency can be prevented through a diet rich in animal products, such as liver and eggs, or through supplementation with beta-carotene, a precursor to vitamin A found in leafy green vegetables. However, it's essential to note that excessive intake of vitamin A can be toxic, particularly in patients with liver disease.

✓ Correct Answer: C. Vitamin A