What describes an absence of all or part of the iris?

Aniridia refers specifically to the congenital absence of all or part of the iris, which is the colored part of the eye that regulates the size of the pupil and thus controls the amount of light entering the eye. This condition can lead to various visual complications because the iris plays a crucial role in vision. Its absence can affect light sensitivity and may lead to associated conditions such as cataracts or glaucoma.

The other terms listed describe different eye conditions. Heterochromia refers to a condition where a person has differently colored eyes or segments of eyes, which is unrelated to the absence of the iris. A low vision patient has reduced vision that cannot be fully corrected with glasses or contact lenses but does not imply structural concerns with the eye itself. A monocular patient is someone who has vision in only one eye, which does not specifically involve the iris or its absence. Thus, aniridia is the only term that directly describes a condition involving the absence of all or part of the iris.