During an ophthalmic procedure, if a drug was mistakenly used that caused dilation of the pupil, which drug was likely administered?

The drug known for causing dilation of the pupil, or mydriasis, is tropicamide. This medication is classified as a muscarinic antagonist. When applied within the eye, it blocks the action of acetylcholine on muscarinic receptors in the iris sphincter muscle, leading to relaxation of this muscle, which allows the pupil to dilate.

Understanding the mechanism of tropicamide clarifies why it would be the likely choice in this scenario. Its rapid onset and shorter duration of action make it particularly useful for ophthalmic examinations requiring pupil dilation.

In contrast, acetylcholine and bethanechol are cholinergic agents that would constrict the pupil (miosis) due to their stimulation of the sphincter muscle and ciliary body. Pilocarpine, also a cholinergic agent, works in a similar manner, promoting pupil constriction. Therefore, these drugs would not produce the desired effect of dilation in this context.