Which medication is most likely causing constipation in a patient taking multiple antihypertensives?

Verapamil is a calcium channel blocker and is known to cause constipation as a common side effect. This occurs because calcium channel blockers can decrease gastrointestinal motility by relaxing smooth muscle in the intestines. In patients taking multiple antihypertensives, especially if verapamil is included, it is likely contributing to gastrointestinal side effects such as constipation.

The other medications listed have different mechanisms of action and are less commonly associated with constipation. For instance, chlorthalidone is a thiazide diuretic that may cause electrolyte imbalances but not typically constipation. Aspirin, while it can affect the gastrointestinal tract and cause issues such as irritation or ulcers, does not usually lead to constipation. Lisinopril is an ACE inhibitor that is primarily used to manage hypertension and heart failure, and constipation is not a common side effect associated with it. Therefore, verapamil stands out as the medication most likely causing constipation among those listed.