Which eye drop inactivates the sphincter muscle of the iris?

The eye drop that inactivates the sphincter muscle of the iris is a cycloplegic agent. Cycloplegics work by paralyzing the ciliary muscle, which is responsible for lens accommodation. In doing so, they also affect the iris sphincter muscle, leading to pupil dilation (mydriasis) and inhibition of the sphincter’s ability to constrict the pupil.

This effect is particularly useful in certain diagnostic procedures or during ophthalmic examinations, allowing for a better view of the retina and other internal structures without the interference of spontaneous pupillary movement. Cycloplegics are frequently used in patients undergoing refraction assessments or when a thorough evaluation of the posterior segment is necessary.

In contrast, the other options do not serve this function: miotic agents constrict the pupil, myopic does not refer to a type of eye drop, and vasodilators target blood vessels rather than the muscles of the iris. Thus, cycloplegics are uniquely identified by their specific action on the iris sphincter muscle.