**Core Concept**
The question tests the understanding of **visceral innervation** and **nociception** in the gastrointestinal tract, specifically the **small bowel**. Visceral pain is mediated by visceral afferent fibers that transmit signals to the central nervous system.

**Why the Correct Answer is Right**
Pain in the small bowel is typically appreciated when it is **distended** or **stretched**, which activates mechanoreceptors and nociceptors in the bowel wall. This can occur due to various reasons such as **obstruction**, **inflammation**, or **ischemia**. The exact mechanism involves the release of **neurotransmitters** and **inflammatory mediators** that stimulate visceral afferent fibers.

**Why Each Wrong Option is Incorrect**
**Option A:** Is not a common cause of pain in the small bowel.
**Option B:** Similarly, is not typically associated with pain in this context.
**Option D:** While it can cause discomfort, it is not the most direct cause of pain.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that visceral pain is often **diffuse** and **poorly localized**, which can make diagnosis challenging.

**Correct Answer:** D. distended.