**Core Concept**
TURP syndrome is a complication of Transurethral Resection of the Prostate (TURP) that can cause periumbilical pain, among other symptoms. This pain is often referred due to irritation of the peritoneum. The mechanism involves absorption of irrigation fluid into the bloodstream.

**Why the Correct Answer is Right**
The commonest cause of periumbilical pain after TURP is the absorption of irrigation fluid, usually glycine or sterile water, into the bloodstream. This leads to dilutional hyponatremia, which irritates the peritoneum and causes pain. The peritoneum is sensitive to changes in osmolality and pH, and the absorbed fluid can cause a chemical peritonitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Intraoperative hypotension can occur with spinal anesthesia, but it is not directly related to periumbilical pain after TURP.
**Option B:** Bupivacaine is the local anesthetic used for spinal anesthesia, but it is not the cause of periumbilical pain in this scenario.
**Option C:** Urethral injury is a possible complication of TURP, but it is not the most common cause of periumbilical pain after the procedure.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing TURP syndrome is to suspect it in any patient who has undergone TURP and presents with periumbilical pain, nausea, vomiting, and hyponatremia. The treatment involves stopping the procedure, administering 3% sodium chloride, and considering hemodialysis if severe hyponatremia is present.

**Correct Answer: C. Urethral injury**