Which mydriatic agent has rapid onset and short duration?

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

Answer: (B)

Rapid onset with short duration is a hallmark of tropicamide among common mydriatics. Tropicamide quickly blocks muscarinic receptors in the iris sphincter, producing pupil dilation with minimal impact on accommodation and a relatively brief action—usually just a few hours. This makes it ideal when you want dilation for an exam but want patients to regain normal vision quickly afterward.

Cyclopentolate also causes dilation but takes longer to act and lasts much longer, often with more cycloplegia, so it doesn’t fit the rapid-onset, short-duration requirement. Atropine has a very long duration, lasting days, which clearly doesn’t fit. Phenylephrine can dilate the pupil promptly, but its effect tends to last longer than tropicamide and it mainly stimulates the iris dilator without the same degree of cycloplegia, so it isn’t as short-acting.

Thus tropicamide best meets the need for rapid onset and short duration.