A visual defect caused by an irregular curvature of the cornea or lens, resulting in blurred vision:

A. hyperopia

B. myopia

C. strabismus

D. astigmatism

D. astigmatism

Explanation:

Astigmatism is an imperfection in the curvature of the eye. Most of the focusing power in the eye occurs along the frontal surface called the cornea. The next structure involved in focusing is the lens, which lies behind the iris inside the eye.

The ideal cornea has a symmetrically curved surface, like a basketball. Astigmatism is caused by a cornea or lens with a curve that is not symmetrical. As a result, people with astigmatism may have distorted or blurred vision.

Astigmatism can occur in families and in most cases is associated with other refractive problems such as nearsightedness or farsightedness. It may also increase over time due to age.