**Core Concept**
The cornea is a complex, multi-layered structure composed of epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. Each layer has distinct characteristics and functions. The question focuses on the metabolically active layer of the cornea, which is crucial for maintaining corneal transparency and function.

**Why the Correct Answer is Right**
The metabolically active layer of the cornea is the **endothelium**. This single layer of cells is responsible for regulating the corneal water and electrolyte balance, maintaining corneal clarity, and pumping excess fluid out of the cornea. The endothelial cells express aquaporin-1 channels, which facilitate water transport, and Na+/K+-ATPase pumps, which maintain the ionic balance. The endothelium's metabolic activity is essential for corneal homeostasis.

**Why Each Wrong Option is Incorrect**
* **Option A:** The endothelium is not the correct answer, but it's the correct answer in this case. We're looking for the metabolically active layer, which is indeed the endothelium.
* **Option B:** The stroma is the thickest layer of the cornea, composed of collagen and keratan sulfate, but it is not metabolically active.
* **Option C:** Descemet's membrane is a thin, acellular layer produced by the endothelium, but it is not metabolically active itself.
* **Option D:** The epithelium is the outermost layer of the cornea, responsible for protecting the eye and maintaining the corneal surface, but it is not the metabolically active layer.

**Clinical Pearl / High-Yield Fact**
The endothelium is responsible for maintaining corneal clarity, and its dysfunction can lead to corneal edema and vision loss. In Fuchs' endothelial corneal dystrophy, the endothelium undergoes atrophy, leading to corneal edema and vision loss.

**Correct Answer:** A. Endothelium