**Core Concept**
The urge to defecate is triggered by the increase in rectal pressure due to the accumulation of fecal matter. This is mediated by the rectoanal inhibitory reflex, which is a complex neural mechanism involving the enteric nervous system, the pelvic nerves, and the spinal cord.

**Why the Correct Answer is Right**
When the rectal wall is distended by a certain amount of feces, the rectoanal inhibitory reflex is activated. This reflex causes the internal anal sphincter to relax, increasing the rectal pressure and triggering the urge to defecate. The sensory receptors in the rectal wall, particularly the stretch receptors, play a crucial role in detecting the increase in rectal pressure and transmitting the signal to the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the exact pressure threshold required to trigger the urge to defecate.
**Option B:** This option is incorrect as it is not a direct measure of the pressure increase in the rectum.
**Option C:** This option is incorrect as it is not a recognized unit of measurement for rectal pressure.

**Clinical Pearl / High-Yield Fact**
The rectoanal inhibitory reflex is a crucial mechanism for maintaining continence and preventing involuntary defecation. Understanding this reflex is essential for diagnosing and managing conditions such as fecal incontinence and constipation.

**Correct Answer:** 80-100 mmHg. (Note: This value may vary slightly depending on the source, but 80-100 mmHg is a commonly cited range for the threshold pressure that triggers the urge to defecate.)