Which adrenergic agent is used for dilation and is cautioned in patients taking monoamine oxidase inhibitors (MAOIs)?

• A. Tropicamide
• B. Phenylephrine
• C. Scopolamine
• D. Apraclonidine

Answer: (B)

Phenylephrine is a direct-acting alpha-1 adrenergic agonist that dilates the pupil by stimulating the radial iris dilator muscle, producing mydriasis without cycloplegia. In patients taking MAO inhibitors, caution is advised because MAOIs raise levels of endogenous catecholamines and can amplify the blood pressure and heart rate–raising effects of adrenergic agonists, increasing the risk of hypertensive crises. The other options either work by different mechanisms (tropicamide and scopolamine are antimuscarinics that cause dilation, not through adrenergic stimulation) or are not primarily used to produce dilation (apraclonidine is an alpha-2 agonist mainly for lowering intraocular pressure). Thus, phenylephrine is the dilation agent that requires caution with MAOIs.