A 72-Year-Old Man Comes to the Office for Evaluation of Constipation

A 72-year-old man comes to the office for evaluation of constipation.  His stools have been hard and pelletlike for as long as he can remember.  The patient has a bowel movement every 3-4 days and frequently strains when using the bathroom.  He has had associated abdominal discomfort but no hematochezia, melena, vomiting, or unexpected weight changes.  The symptoms have not improved despite fiber supplementation.  Vital signs are within normal limits.  The abdomen is mildly distended with decreased bowel sounds.  In addition to increasing water consumption, the patient is advised to try bisacodyl for constipation.  What is the primary mechanism of action of this medication?

A) Decreases surface tension of stool
B) Draws water into the stool by creating an osmotic gradient
C) Draws water into the stool by directly activating chloride channels
D) Improves peristalsis by blocking mu-receptors
E) Improves peristalsis by stimulating enteric nerves