Drugs that dilate the pupil without cycloplegia operate by stimulating which iris muscle?

The correct answer is based on the understanding of the anatomy of the iris and its muscles. The iris contains two primary types of muscles that regulate pupil size: the sphincter muscle and the dilator muscle. The dilator muscle, when stimulated, causes the pupil to widen, or dilate, without causing paralysis of the ciliary muscle, which is responsible for accommodation and also impacts the shape of the lens for focusing.

When drugs cause pupil dilation without cycloplegia, they are specifically targeting the dilator muscle. This action allows the pupil to open wider without affecting the ability to focus on nearby objects, which is what cycloplegia would do by paralyzing the ciliary muscle.

Understanding these muscles is vital in pharmacology and ophthalmology, as different agents can selectively influence these muscles to achieve desired effects in treatment. The importance of this distinction helps guide therapeutic choices, especially in conditions requiring pupil dilation for examination or treatment without compromising visual accommodation.