What is described as a true opacity that interferes with vision?

Cataracts are characterized as true opacities that form within the lens of the eye, obstructing the passage of light and thus interfering with vision. They often result from a variety of factors, including age, genetics, environmental influences, or underlying medical conditions. When a cataract develops, it creates a cloudiness in the lens, which can significantly reduce visual acuity and lead to symptoms such as blurriness, glare, and difficulty seeing at night.

Nuclear sclerosis, while it does cause a change in the lens, is an age-related condition that results in a hardening of the lens nucleus rather than an opacity that obstructs light. Intumescent cataracts refer to a specific type of cataract that is swollen and may appear clear or slightly cloudy without impairing vision to a significant degree initially. Lens subluxation is a condition where the lens is partially dislocated, which may also affect vision but is not described as a true opacity. Thus, cataracts are the correct choice for this question as they represent a direct cause of visual obstruction due to their opacity.