What is the defining characteristic of a pterygium?

A pterygium is specifically characterized by its appearance as a triangular-shaped growth of conjunctival tissue that extends onto the cornea. This condition is often associated with exposure to UV light and can be found on the nasal side of the eye, gradually advancing towards the pupil. The defining feature is its triangular shape, which helps distinguish it from other eye conditions that may present similarly but lack this distinct geometry.

The other options describe symptoms that may occur with various eye conditions but do not specifically represent a pterygium. For instance, a cloudy cornea can indicate several issues, such as cataracts or corneal scarring, whereas swollen eyelids may be associated with allergic reactions or infections, and a red sclera typically signifies conjunctivitis or episcleritis. These characteristics are not intrinsic to a pterygium, thus underlining why the triangular conjunctival growth is the hallmark of this condition.