**Core Concept**
The corneal endothelium is a single layer of cells lining the posterior surface of the cornea, playing a crucial role in maintaining corneal clarity by regulating fluid and electrolyte balance. This layer is essential for the proper functioning of the cornea.

**Why the Correct Answer is Right**
The corneal endothelium is derived from the mesenchyme, a type of embryonic connective tissue. During embryonic development, the mesenchyme gives rise to various structures, including the corneal endothelium. The endothelial cells originate from the neural crest, a group of cells migrating from the dorsal neural tube, and then differentiate into corneal endothelial cells. This process occurs during the sixth to eighth weeks of gestation.

**Why Each Wrong Option is Incorrect**
**Option A:** The epithelium, not the endothelium, is derived from the surface ectoderm. The corneal epithelium is the outermost layer of the cornea and serves as a barrier against external factors.
**Option B:** The stroma, the middle layer of the cornea, is composed of collagen and keratocytes, but it is not derived from the same embryonic origin as the endothelium. The stroma is produced by the differentiation of mesenchymal cells.
**Option C:** The iris and ciliary body are derived from the neuroectoderm, but not the corneal endothelium. These structures are responsible for controlling pupil size and lens accommodation.

**Clinical Pearl / High-Yield Fact**
The corneal endothelium has limited regenerative capacity, making it essential to protect this layer from injury or disease. Factors such as aging, trauma, and certain medical conditions can lead to endothelial dysfunction, resulting in corneal edema and vision loss.

**Correct Answer: C.