A patient has pupillary dilation; which drug would most likely cause this effect?

• A. Epinephrine
• B. Pilocarpine
• C. Atropine
• D. Acetaminophen

Answer: (C)

Pupillary size is governed by the tug-of-war between parasympathetic constriction and sympathetic dilation of the iris muscles. Atropine is an anticholinergic that blocks muscarinic receptors on the iris sphincter, so the parasympathetic signal to constrict is halted. With that constriction removed, the sympathetic action on the radial dilator muscle predominates, leading to a dilated pupil.

Pilocarpine, by contrast, is a muscarinic agonist that promotes constriction (miosis), not dilation. Acetaminophen doesn’t affect pupil size. Epinephrine can cause some dilation via sympathetic pathways, but blocking the constricting signal with atropine provides a more direct and robust mechanism for dilation.