What mechanism do osmotic laxatives use to promote bowel movement?

Osmotic laxatives work by drawing water into the intestines, which leads to fluid accumulation. This increase in fluid within the bowel causes the stool to soften and become bulkier, effectively distending the bowel. The distension encourages peristalsis—the wave-like muscle contractions of the intestines—promoting a bowel movement. This mechanism is particularly effective because the added water not only eases the passage of stool but also stimulates the bowel's natural movements in response to the stretching of its walls.

The other options involve mechanisms that do not pertain to how osmotic laxatives function. For instance, increasing fiber content emphasizes dietary changes rather than a pharmacological approach. Similarly, stimulating nerve endings pertains more to medications that affect nerve activity in the gastrointestinal system, and increasing muscle contraction relates to stimulant laxatives, which work differently. Thus, the accumulation of fluid leading to bowel distension correctly identifies the primary action of osmotic laxatives.