Question 7

Question:

A patient is administered a pharmacologic agent topically onto the eye, resulting in mydriasis without cycloplegia. Which of the following agents was administered?

Correct Answer: D. Phenylephrine

The anterior chamber of the eye is controlled primarily by three muscles (pupillary constrictors, pupillary dilators, and ciliary muscle). Activation of constrictors (through muscarinic receptors) leads to miosis/constriction; activation of dilators (through alpha-adrenergic receptors) leads to mydriasis/dilation; and activation of the ciliary muscle (through muscarinic receptors) allows accommodation. Bethanechol and neostigmine result in increased ACh & muscarinic activation, thus causing miosis in the pt. Atropine would block all muscarinic activation, resulting in not only mydriasis but also cycloplegia (loss of accomodation due to ciliary muscle paralysis). Praloidoxime is an ACh reactivator, so it wouldn’t have any effect here. Therefore, phenylephrine–a selective alpha1-adrenergic agonist–is the only viable possibility.