Which drug is used to dilate the pupil during an equine ocular examination?

• A. Tropicamide 1%
• B. Proparacaine
• C. Carbocaine
• D. Xylazine

Answer: (A)

Pupil dilation for an equine ocular exam is achieved with a drug that blocks parasympathetic constriction of the iris, allowing the pupil to widen quickly and safely. Tropicamide 1% is a short-acting antimuscarinic agent that does just that by blocking muscarinic receptors on the iris sphincter. It produces mydriasis with a rapid onset—typically within 15–30 minutes—and a duration of a few hours, which is ideal for an exam because you get a clear view of internal structures without prolonged impairment. It also has the advantage of fewer systemic effects and less cycloplegia than atropine, making it a preferred choice in horses.

The other substances in the list serve different purposes: proparacaine is a topical anesthetic used to numb the cornea during examination and does not dilate the pupil; carbocaine refers to lidocaine, another local anesthetic; xylazine is a sedative/analgesic that can cause some pupil dilation as a secondary effect but is not used as a routine dilating agent due to systemic effects and variability in response.