**Core Concept**
Vitamin A plays a crucial role in maintaining the integrity and function of the epithelial lining in the respiratory and gastrointestinal tracts. Its deficiency can lead to impaired epithelial barrier function, increasing susceptibility to infections.

**Why the Correct Answer is Right**
The correct answer is C. Dysfunction of epithelial layer. Vitamin A is essential for the normal differentiation and maintenance of epithelial cells. In its deficiency, the epithelial lining becomes compromised, leading to impaired mucociliary clearance, increased susceptibility to infections, and reduced immune response. The epithelial layer dysfunction impairs the mucociliary clearance mechanism, allowing pathogens to colonize and cause infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Keratinization of upper epithelial cells is a characteristic feature of Vitamin A deficiency, but it is not the primary reason for increased susceptibility to respiratory tract infections. Keratinization leads to a hardened, scaly skin and mucosal surfaces, but it does not directly impair the epithelial barrier function.

**Option B:** Lack of antibody production is not a direct consequence of Vitamin A deficiency. While Vitamin A is essential for the proper functioning of the immune system, its deficiency primarily affects the epithelial barrier function and mucociliary clearance, rather than antibody production.

**Option D:** Defective chemotaxis is not a direct consequence of Vitamin A deficiency. Chemotaxis refers to the movement of cells towards a chemical stimulus, and while Vitamin A is essential for the proper functioning of the immune system, its deficiency primarily affects the epithelial barrier function and mucociliary clearance.

**Clinical Pearl / High-Yield Fact**
Vitamin A deficiency can lead to a range of clinical manifestations, including night blindness, dry skin, and impaired wound healing. In addition to increased susceptibility to respiratory tract infections, Vitamin A deficiency can also lead to impaired mucociliary clearance and increased susceptibility to gastrointestinal infections.

**✓ Correct Answer: C. Dysfunction of epithelial layer**