**Core Concept**
The question is testing the student's knowledge of the anatomy and spread of acute abdominal pain. The retroperitoneal space is a potential space in the abdominal cavity that contains various vital structures such as the kidneys, pancreas, and major blood vessels. The spread of acute abdominal pain to the retroperitoneal space suggests involvement of these structures.

**Why the Correct Answer is Right**
The correct answer is likely to be the pancreas, as it is a retroperitoneal organ that can cause severe abdominal pain when inflamed or infected. The pancreas is surrounded by a fibrous capsule and is located in the retroperitoneal space, making it vulnerable to inflammation and infection. The pain from pancreatic pathology can radiate to the back and is often severe.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the stomach can cause abdominal pain, it is not typically located in the retroperitoneal space and is less likely to cause back pain.
* **Option B:** The liver is not typically located in the retroperitoneal space, but rather in the peritoneal cavity. While liver pathology can cause pain, it is less likely to radiate to the back.
* **Option D:** The small intestine can cause abdominal pain, but it is not typically located in the retroperitoneal space and is less likely to cause back pain.

**Clinical Pearl / High-Yield Fact**
The "Cullen's sign" and "Grey Turner's sign" are classical signs of pancreatic pathology, with the former referring to a bluish discoloration of the umbilicus and the latter referring to bruising of the flanks. These signs are often seen in patients with acute pancreatitis.

**Correct Answer:** C.