Which drug is associated with a red cap and a duration of about 3 hours?

• A. Phenylephrine
• B. Tropicamide
• C. Cyclopentolate
• D. Homatropine

Answer: (A)

In this area, the color of the cap and how long the eye remains dilated help identify the dilating drop. A red-cap bottle points to phenylephrine, a direct-acting alpha-adrenergic agonist that dilates the pupil by stimulating the iris dilator muscle. Its effect is relatively short, typically lasting about three hours (often a bit longer depending on concentration), which matches the described duration. It causes little to no cycloplegia, so the eye’s focusing ability remains mostly intact, unlike some other dilating drugs that produce stronger cycloplegia and last longer.

Other common dilating drops have longer or shorter effects and different levels of cycloplegia: tropicamide lasts around four to six hours with mild cycloplegia; cyclopentolate lasts about a day or more with more pronounced cycloplegia; and homatropine can last for days with significant cycloplegia. Therefore, the combination of a red cap and a duration of around three hours best fits phenylephrine.