Which type of eye drop is used to paralyze the sphincter muscle of the iris?

The correct answer is cycloplegic eye drops, which are specifically designed to paralyze the sphincter muscle of the iris and also the ciliary muscle. When the sphincter muscle is paralyzed, it prevents constriction of the pupil, leading to dilation. This is particularly useful during certain eye examinations and procedures, as it allows for better visualization of the internal structures of the eye.

Cycloplegic agents function by blocking the action of acetylcholine on the muscles of the iris and ciliary body, effectively halting both pupil constriction and accommodation. This action is essential for thoroughly assessing refractive errors and diagnosing various ocular conditions.

In contrast, mydriatic drops primarily focus on dilation by acting on the dilator muscle of the iris but do not affect accommodation. Antihistamines are typically used to alleviate allergy symptoms and have no direct role in affecting pupil size or muscle paralysis. Beta-blockers are primarily used to lower intraocular pressure in conditions like glaucoma and are not indicated for paralyzing muscles of the iris.