Final answer:
An agent that causes pupil dilation is known as a mydriatic, with phenylephrine being an example used during eye exams.
Step-by-step explanation:
An agent that causes the pupil to dilate is called a mydriatic. Mydriasis, or the dilation of the pupil, can be induced by sympathetic stimulation, where norepinephrine binds to adrenergic receptors in the radial fibers of the iris smooth muscle. The drug phenylephrine mimics the action of norepinephrine by binding to the same receptors, which is why it is used in a doctor's office to dilate pupils during an eye exam.