**Core Concept**
The lens in the eye is a critical structure for focusing light on the retina, and its development is closely tied to the embryological formation of the anterior segment. The lens is formed from the surface ectoderm and the optic vesicle, which interact to give rise to the lens placode.

**Why the Correct Answer is Right**
The lens placode is a thickening of the surface ectoderm that invaginates to form the lens pit. This process is regulated by the interaction between the surface ectoderm and the optic vesicle, which express specific transcription factors and signaling molecules that guide lens development. As the lens pit deepens, it separates from the surface ectoderm to form the lens vesicle, which eventually differentiates into the mature lens.

**Why Each Wrong Option is Incorrect**
**Option A:** The cornea is derived from the surface ectoderm, but it is not the source of the lens.
* The cornea is a transparent layer covering the anterior surface of the eye, whereas the lens is a transparent, biconvex structure behind the iris.

**Option B:** The retina is derived from the optic cup, but it is not the source of the lens.
* The retina is the innermost layer of the eye, responsible for light detection and signal transduction, whereas the lens is a refractive structure that focuses light on the retina.

**Option C:** The iris is derived from the mesenchyme surrounding the optic cup, but it is not the source of the lens.
* The iris is a pigmented, circular structure controlling light entry into the eye, whereas the lens is a clear, refractive structure behind the iris.

**Clinical Pearl / High-Yield Fact**
The lens is the only part of the eye that is not derived from the optic cup, highlighting the distinct developmental origins of the different eye structures.

**Correct Answer: C. The iris is derived from the mesenchyme surrounding the optic cup, but it is not the source of the lens.**