Which eye drop inactivates the sphincter muscle of the iris?

The cycloplegic eye drop is designed to temporarily paralyze the ciliary muscle of the eye, which can also affect the sphincter muscle of the iris. This paralysis leads to cycloplegia, meaning a loss of the eye's ability to focus on near objects, as well as dilation of the pupil, or mydriasis. The resulting effect means that the iris can no longer constrict in response to bright light or focus adjustments, which is why cycloplegic droplets are often used in various ophthalmic examinations or procedures.

Myotic drops, by contrast, are intended to cause constriction of the pupil and would not inactivate the sphincter muscle; rather, they would activate it. Antihistamines are primarily used to address allergic reactions and do not have a direct effect on the muscle action of the iris, while vasodilators focus more on blood vessel dilation rather than affecting the iris muscles specifically. Cycloplegics are thus the correct choice for this question as they specifically induce a state that inactivates the sphincter muscle of the iris.