**Core Concept:** Floaters are small opacities or shadows within the vitreous humor of the eye, which appear as moving dots or strings when the eye moves. They are most commonly caused by the fragmentation and degeneration of retinal pigment cells and are usually benign. The vitreous humor is a clear, gel-like substance that fills the space between the lens and the retina.

**Why the Correct Answer is Right:** Option D is the correct answer because floaters are not typically seen in the cornea. The cornea is the transparent, dome-shaped outer covering of the eye that protects and refracts light entering the eye. Floaters are not present in the cornea as it lacks the cellular components and pigment cells that contribute to the formation of floaters in the vitreous humor.

**Why Each Wrong Option is Incorrect:** Option A, B, and C are incorrect because floaters are a common occurrence in the vitreous humor, a gel-like substance within the eye. Floaters are formed by the accumulation of pigment debris, clumps, or membranes from degenerating retinal pigment cells. These obstructions block the passage of light, appearing as floaters when the eye moves.

**Clinical Pearl:**

Occasionally, the sudden appearance of numerous floaters may indicate a serious intraocular condition, such as a vitreous hemorrhage, retinal tear, or detachment. In such cases, it is essential to seek immediate medical attention to avoid potential vision loss or blindness.

**Correct Answer:** D (cornea)