Which of the red-cap drops has the longest duration, approximately 1-2 days?

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

Answer: (A)

The question tests how long each common dilating drop lasts. Among these agents, Homatropine typically produces dilation with cycloplegia that lasts about 24 to 72 hours, often around one to two days. This longer duration arises because Homatropine is a relatively potent antimuscarinic that blocks the muscarinic receptors in the iris sphincter and ciliary muscle, causing sustained dilation and loss of accommodation. In contrast, phenylephrine dilates by stimulating the iris dilator muscle and tends to wear off in several hours; tropicamide is a short-acting antimuscarinic with effects lasting only about 4–6 hours; cyclopentolate lasts longer than tropicamide but usually only about a day to a day and a half. So the drop associated with a dilation lasting roughly one to two days is Homatropine.