**Core Concept**
TURP syndrome, a complication of transurethral resection of the prostate (TURP), involves fluid overload and absorption of irrigation fluid, leading to electrolyte imbalances and periumbilical pain. This occurs due to the absorption of the irrigation fluid through the resected prostate tissue.

**Why the Correct Answer is Right**
The commonest cause of periumbilical pain after TURP under spinal anesthesia with Bupivacaine is TURP syndrome. This occurs due to the absorption of the irrigation fluid, which contains hypotonic solutions like glycine or sorbitol, leading to a shift of fluid into the vascular space. The resulting fluid overload causes periumbilical pain, nausea, and vomiting, as well as electrolyte imbalances like hyponatremia.

**Why Each Wrong Option is Incorrect**
**Option A:** While Bupivacaine can cause hypotension, it is not the primary cause of periumbilical pain in this scenario.

**Option B:** Spinal anesthesia is generally safe for TURP procedures, and its use does not directly contribute to the development of TURP syndrome.

**Option C:** While electrolyte imbalances can occur in TURP syndrome, they are not the primary cause of periumbilical pain.

**Clinical Pearl / High-Yield Fact**
To prevent TURP syndrome, it is essential to use a low-pressure irrigation system and monitor the patient's fluid status closely during the procedure.

**Correct Answer:** C.