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

The correct answer is the dilator muscle, which is responsible for pupil dilation. When the dilator muscle contracts, it pulls the iris taut, effectively widening the pupil to allow more light into the eye. This process is associated with the sympathetic nervous system, which is activated under low light conditions or in response to certain stimuli, leading to the dilation of the pupil without causing paralysis of the ciliary muscle, which is known as cycloplegia.

This action contrasts with the sphincter muscle of the iris, which constricts the pupil and is activated by the parasympathetic nervous system. The constrictor muscle also refers to this same sphincter function and is responsible for reducing the size of the pupil when there is bright light or when focusing on near objects. The radial muscle does not accurately describe a specific muscle associated with this action; rather, it refers to the same dilator muscle concept under different nomenclature.

Focusing on pupil dilation specifically, the term "dilation" directly indicates the role of the dilator muscle, making this choice the most appropriate for the scenario described.