What characterizes the condition of keratoconus?

Keratoconus is primarily characterized by the thinning and bulging of the cornea. In this condition, the normally round dome-shaped cornea gradually thins and develops a conical shape, which can lead to significant visual impairment. As the cornea protrudes outward, it distorts light entering the eye, resulting in blurred vision and increased sensitivity to light and glare. The changes in the cornea are progressive, and while this can lead to severe vision issues if left untreated, the unique deformation is a defining feature of keratoconus.

The other conditions described in the other choices involve entirely different aspects of eye health. For example, inflammation of the optic nerve pertains to conditions like optic neuritis, while gradual loss of peripheral vision is more closely related to glaucoma or retinal diseases. Discoloration of the lens is indicative of cataracts. Thus, the distinctive thinning and bulging are what specifically defines keratoconus among these options.