**Core Concept:** Pelvic splanchnic nerves are a group of nerves that supply abdominal organs, specifically the organs in the pelvic cavity and abdominal cavity. They are branches of the lumbar nerves within the sympathetic chain.

**Why the Correct Answer is Right:** The correct answer, **D**, refers to the diaphragm as the structure not supplied by pelvic splanchnic nerves. The diaphragm is a muscle that separates the thoracic cavity from the abdominal cavity. It is innervated by the phrenic nerves, which are derived from the C3-C5 spinal segments and are part of the ventral rami of the cervical nerves.

**Why Each Wrong Option is Incorrect:**

A. The rectus abdominis muscle (Option A) is innervated by the lumbar splanchnic nerves, which are branches of the lumbar nerves within the sympathetic chain.

B. The urinary bladder (Option B) receives its innervation from the pelvic splanchnic nerves, which supply the bladder, urethra, and ureter.

C. The small intestine (Option C) is supplied by the superior and inferior mesenteric arteries, which are branches of the abdominal aorta, and the small intestine is also supplied by the lumbar splanchnic nerves, not pelvic splanchnic nerves.

E. The colon (Option E) is supplied by the superior and inferior mesenteric arteries, which are branches of the abdominal aorta, not the pelvic splanchnic nerves.

**Clinical Pearl:** Understanding the anatomy and innervation of abdominal organs is crucial in patients with abdominal pain or dysfunction, as it helps differentiate between different pain patterns and diagnose potential nerve injuries or neuropathies.

**Correct Answer:** D (diaphragm)