What causes mydriasis in patients?

Mydriasis, or dilation of the pupils, occurs when there is an increase in sympathetic nervous system activity or a decrease in parasympathetic activity affecting the iris sphincter muscle. Anti-cholinergics, specifically, block the action of acetylcholine on the muscarinic receptors in the eye. This inhibition reduces the tone of the sphincter muscle, leading to pupil dilation.

The other options do not have a primary mechanism that leads to mydriasis. Beta-blockers primarily affect the cardiovascular system by blocking adrenergic receptors, and while they may have some effect on ocular pressure, they do not directly cause pupil dilation. Calcium channel blockers affect vascular smooth muscle and cardiac tissue but do not have a direct effect on pupil size. SSRIs primarily target serotonin reuptake in the brain and are associated mainly with mood regulation and do not cause pupil dilation as a primary effect. Thus, the action of anti-cholinergics is distinctly responsible for causing mydriasis.