Which of the following is a primary stimulus for defecation?

• A. Absorption of nutrients in the small intestine
• B. Secretion of gastric acid in the stomach
• C. Release of bile from the gallbladder
• D. Distension of the rectum by feces

Answer: (D)

Verified step by step guidance

1. Understand the process of defecation: Defecation is the elimination of feces from the rectum through the anus. It is primarily controlled by the nervous system and involves both voluntary and involuntary mechanisms.
2. Identify the primary stimulus for defecation: The rectum is a muscular tube that stores feces temporarily. When feces accumulate in the rectum, they cause distension (stretching) of the rectal walls, which serves as the primary stimulus for defecation.
3. Explain the role of rectal distension: Distension of the rectum activates stretch receptors in the rectal walls. These receptors send signals to the spinal cord, initiating the defecation reflex, which involves relaxation of the internal anal sphincter and contraction of the rectal muscles.
4. Clarify why other options are incorrect: Absorption of nutrients in the small intestine, secretion of gastric acid in the stomach, and release of bile from the gallbladder are unrelated to the defecation process. These functions are part of digestion and nutrient absorption, not waste elimination.
5. Summarize the correct answer: The primary stimulus for defecation is the distension of the rectum by feces, which triggers the defecation reflex and facilitates the elimination of waste.