**Core Concept**
Cullen's sign is a clinical manifestation of severe intra-abdominal infection, specifically pancreatic necrosis, which presents as a bluish discoloration of the periumbilical skin.

**Why the Correct Answer is Right**
Cullen's sign is caused by the tracking of hemorrhagic pancreatic fluid to the periumbilical region through the fascial planes and lymphatic vessels. This process is facilitated by the absence of a peritoneal reaction, which is often seen in severe infections. The bluish discoloration is due to the presence of melanin in the blood, which is broken down to form melanin pigments in the skin. This sign is a classic indicator of severe pancreatic disease and requires prompt surgical intervention.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might refer to Grey-Turner's sign, which is a similar bluish discoloration of the flanks, but it is not the correct answer for Cullen's sign.
**Option B:** This option might refer to a skin rash or infection, but it does not accurately describe the pathophysiology of Cullen's sign.
**Option C:** This option might refer to a skin lesion or tumor, but it is not related to the intra-abdominal infection causing Cullen's sign.

**Clinical Pearl / High-Yield Fact**
Cullen's sign is a rare but significant clinical finding that requires immediate surgical evaluation and intervention to prevent further complications.

**Correct Answer: A. Cullen's sign is a bluish discoloration of the periumbilical skin due to intra-abdominal infection.**