Which eye drop inactivates the sphincter muscle of the iris?

The sphincter muscle of the iris controls the constriction of the pupil. When this muscle is inactivated, the pupil dilates, allowing more light to enter the eye. Cycloplegics are a category of medications that cause paralysis of the ciliary muscle, which leads to mydriasis (pupil dilation) and cycloplegia (paralysis of accommodation).

In the context of this question, the correct answer indicates that cycloplegics effectively interfere with the sphincter muscle's function, thus resulting in the dilation of the pupil. This is particularly useful in clinical situations that require examination of the inside of the eye or during certain surgical procedures, as it allows easier access to the retina by preventing the muscle from constricting.

Other options listed are not primarily designed to inactivate the sphincter muscle; for instance, pilocarpine typically causes constriction of the pupil, while tropicamide and atropine, although they can cause dilation, are not categorically classified as "cycloplegics" in the same context. Tropicamide, for example, is often used for diagnostic purposes due to its short duration of action, while atropine is a longer-acting agent with distinct applications.