**Core Concept**
The C loop of the duodenum widens due to the invasion of the duodenum by a pancreatic head mass, which causes a mechanical obstruction and subsequent stretching or widening of the duodenal lumen. This phenomenon is a classic radiological sign associated with pancreatic pathology.

**Why the Correct Answer is Right**
The widening of the C loop of the duodenum is a result of the growth of the pancreatic head, which physically displaces and stretches the duodenum, leading to its characteristic widening. This is often seen in cases of pancreatic carcinoma or adenocarcinoma. The pancreatic head is located in close proximity to the duodenum, and its growth can easily encroach upon the duodenal lumen, causing this radiological sign. The resulting widening of the C loop can be visualized on imaging studies such as CT scans or barium studies.

**Why Each Wrong Option is Incorrect**
**Option B:** Carcinoma of the stomach is unlikely to cause widening of the C loop of the duodenum, as it is located further away from the duodenum and would not directly cause mechanical obstruction or stretching of the duodenum.

**Option C:** Splenic involvement, while it can cause pancreatic pathology, is not directly related to the widening of the C loop of the duodenum. Splenic pathology typically affects the spleen itself and is not directly associated with duodenal widening.

**Option D:** Involvement of the upper renal pole is unrelated to the widening of the C loop of the duodenum, as the kidney and duodenum are separate anatomical structures that do not directly interact.

**Clinical Pearl / High-Yield Fact**
A key radiological sign of pancreatic head pathology is the "double density" sign, which refers to the visualization of the pancreatic head and the surrounding duodenum as two distinct densities on imaging studies.

**✓ Correct Answer: A. Pancreatic head growth. Pancreatic head growth.**