Which condition is listed as a cause of mydriasis?

• A. Drugs
• B. Uveitis
• C. Phthisis
• D. Endotheliitis

Answer: (A)

Mydriasis happens when the iris dilator muscle is activated or when the parasympathetic input to the iris sphincter is blocked. Medications are a classic cause because many drugs either stimulate adrenergic pathways or block acetylcholine at the sphincter muscle, leading to pupil dilation. In horses, topical or systemic anticholinergic agents (which inhibit the sphincter) and adrenergic agonists can produce a dilated pupil, which is why drugs are commonly listed as a cause of mydriasis.

Uveitis tends to cause constriction (miosis) due to iris sphincter spasm and inflammatory mediators; a shrunken globe in phthisis can be associated with a small, nonfunctional pupil rather than dilation; endotheliitis is inflammation of the corneal endothelium and does not typically produce mydriasis. Therefore, drugs are the correct cause among the options.