Which drug is commonly used to dilate pupils for ophthalmic examination?

• A. Pilocarpine
• B. Neostigmine
• C. Atropine
• D. Tropicamide

Answer: (C)

Dilation of the pupil for an eye exam is achieved by blocking the parasympathetic control of the iris. Atropine is a potent muscarinic antagonist, so it stops acetylcholine from acting on the sphincter pupillae muscle. That prevents constriction, allowing the pupil to dilate widely. In addition, atropine causes cycloplegia, paralyzing the ciliary muscle, which helps keep the eye from accommodating during the examination and gives a clearer view of internal structures like the lens and retina. Its effects last longer than many other dilators, which can be advantageous when a thorough, extended examination is needed, though systemic anticholinergic side effects require careful use. Pilocarpine would do the opposite by constricting the pupil, neostigmine would enhance acetylcholine and also promote constriction, and tropicamide is a shorter-acting dilator with less cycloplegia, so atropine's combination of strong dilation, cycloplegia, and longer duration makes it a commonly used choice in many veterinary ophthalmic contexts.