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

The correct answer is that drugs that dilate the pupil without causing cycloplegia operate by stimulating the dilator muscle of the iris. The primary function of the dilator muscle is to widen the pupil, allowing more light to enter the eye when necessary, such as in low-light conditions or during certain medical examinations.

This process is achieved through sympathetic nervous system stimulation, which causes the dilator muscle to contract and pull the iris outward, enlarging the pupil. This action occurs separately from the sphincter muscle, which is responsible for constricting the pupil and does not activate during dilation. Cycloplegia refers to paralysis of the ciliary muscle, affecting accommodation, but in this scenario, dilation occurs without affecting the ability to focus. Thus, identifying the role of the dilator muscle in pupil dilation is crucial for understanding the physiological response involved in various medical scenarios.