**Core Concept**
Epithelialization is a critical phase of wound healing where the epithelial cells migrate, proliferate, and differentiate to cover the wound surface. This process is essential for restoring the skin's integrity and barrier function. The impairment of epithelialization can lead to delayed wound healing and increased susceptibility to infections.

**Why the Correct Answer is Right**
The correct answer is **C. Hypoxia**. Hypoxia impairs epithelialization by reducing the availability of oxygen, which is essential for the energy-intensive process of cell migration and proliferation. Epithelial cells require a sufficient oxygen supply to produce the necessary energy for these processes. In hypoxic conditions, the expression of growth factors, such as vascular endothelial growth factor (VEGF), is also reduced, further impairing epithelialization. The reduced oxygen levels can also lead to an increase in reactive oxygen species (ROS), which can damage the epithelial cells and further impair wound healing.

**Why Each Wrong Option is Incorrect**
**Option A:** **Corticosteroids** actually promote wound healing by increasing the synthesis of growth factors, such as VEGF, and improving the migration and proliferation of epithelial cells.

**Option B:** **Infection** can actually impair epithelialization by damaging the epithelial cells and creating a pro-inflammatory environment, but it is not the correct answer in this context.

**Clinical Pearl / High-Yield Fact**
It's essential to note that hypoxia can be caused by various factors, including smoking, diabetes, and cardiovascular disease, which can all impair wound healing. Therefore, addressing these underlying factors is crucial for promoting wound healing.

**Correct Answer: C. Hypoxia**