**Core Concept**
The cornea's stromal hydration is crucial for maintaining its clarity and mechanical strength. The cornea's unique anatomy allows for precise regulation of stromal hydration, which is vital for its transparency.

**Why the Correct Answer is Right**
The endothelium, a single layer of hexagonal cells, plays a crucial role in maintaining the cornea's hydration balance. It acts as a selective permeability barrier, allowing for the exchange of ions and water while preventing the entry of larger molecules. The endothelium's pump function, mediated by the sodium-potassium pump, helps to remove excess ions from the stroma, thereby reducing water entry and maintaining stromal hydration. This delicate balance is essential for the cornea's transparency and refractive power.

**Why Each Wrong Option is Incorrect**
**Option A:** The Descemet membrane is a thin, acellular layer produced by the endothelium, but it does not directly contribute to stromal hydration regulation.
**Option C:** The epithelium, the outermost corneal layer, helps maintain the cornea's surface integrity and facilitates wound healing but does not directly influence stromal hydration.
**Option D:** The stroma itself is the thick, middle layer of the cornea, composed of collagen fibers and keratocytes, but it does not have the necessary cellular structure or function to regulate its own hydration.

**Clinical Pearl / High-Yield Fact**
In corneal edema, the endothelium's pump function is compromised, leading to increased stromal hydration and corneal thickening. This can result in reduced vision and, in severe cases, corneal ulceration or perforation.

**✓ Correct Answer: B. Endothelium. The endothelium plays a crucial role in maintaining the cornea's hydration balance by regulating ion and water exchange.**