Which ocular structure is primarily affected in cataracts?

Cataracts primarily affect the lens of the eye. This condition involves the clouding of the lens, which is the transparent structure located behind the iris and responsible for focusing light onto the retina. As cataracts develop, they can lead to blurry vision, increased difficulty with glare, and ultimately, a decline in visual acuity.

The lens's primary role is to focus light, so any alteration to its clarity directly impacts vision. In a healthy eye, the lens allows light to pass through unobstructed, aiding in clear vision. When cataracts form, they disrupt this process by scattering and diffusing incoming light, leading to the characteristic symptoms associated with the condition.

Other structures like the cornea, retina, or optic nerve do not undergo changes specific to cataract formation. The cornea is involved in refracting light but remains clear in cataract cases. The retina is responsible for processing visual information but is not affected by the clouding associated with cataracts. The optic nerve transmits visual information from the retina to the brain and is untouched by cataract development. Thus, the lens is the correct answer as the primary structure impacted by cataracts.