Which of the following is commonly used as a mydriatic?

Tropicamide is commonly used as a mydriatic because it has a rapid onset and relatively short duration of action compared to other mydriatics. It works by blocking the action of acetylcholine on the iris sphincter muscle, leading to dilation of the pupil. This property makes tropicamide particularly useful in ophthalmic examinations where quick pupil dilation is needed to allow for a clear view of the retina and other structures in the eye.

In contrast, while phenylephrine is a potent mydriatic agent primarily affecting the dilator pupillae muscle and is effective for increasing pupil size, it is less commonly used in routine exams compared to tropicamide due to its longer onset and potential side effects. Atropine and scopolamine are also mydriatics, but they have a much longer duration of action and are typically used for specific therapeutic purposes, such as treating inflammatory conditions of the eye, rather than for routine dilations needed for examinations. Thus, tropicamide is preferred for its favorable profile in clinical settings.