**Core Concept**
KF ring in Wilson's disease is a characteristic clinical finding related to the pathophysiology of copper accumulation in the cornea. In Wilson's disease, the accumulation of copper leads to damage in various tissues, including the cornea. The KF ring is a specific manifestation of this damage.

**Why the Correct Answer is Right**
The KF ring is seen at the level of Descemet's membrane due to the accumulation of copper in the cornea. This is because the endothelial cells of the cornea are responsible for maintaining the integrity of Descemet's membrane, and copper accumulation leads to damage and thickening of this membrane. Descemet's membrane is a thin layer of tissue between the stroma and the endothelium of the cornea, and its thickening is a hallmark of Wilson's disease. The copper accumulation causes a brownish discoloration of Descemet's membrane, resulting in the characteristic KF ring.

**Why Each Wrong Option is Incorrect**
**Option B:** The epithelium is the outermost layer of the cornea, and while it can be affected in various corneal diseases, it is not directly involved in the formation of the KF ring.
**Option C:** The endothelium is the innermost layer of the cornea, and while it is responsible for maintaining the cornea's clarity, it is not the primary site of copper accumulation in Wilson's disease.
**Option D:** The stroma is the middle layer of the cornea, and while it can be affected in various corneal diseases, the KF ring is specifically related to the damage of Descemet's membrane.

**Clinical Pearl / High-Yield Fact**
The KF ring is a valuable diagnostic clue for Wilson's disease, and its presence should prompt further investigation for this condition. In addition to the KF ring, patients with Wilson's disease may also exhibit Kayser-Fleischer spots in the iris, which are another important diagnostic feature.

✓ Correct Answer: A. Descemet's membrane