**Core Concept**
Gallstone ileus is a rare complication of gallstones where a gallstone enters the intestinal tract and causes a bowel obstruction. On a plain abdominal radiograph, several diagnostic features can be identified to confirm the diagnosis.

**Why the Correct Answer is Right**
The correct answer is **C**. A characteristic feature of gallstone ileus on a plain abdominal radiograph is the presence of a large gallstone in the intestinal lumen, which can be seen as a radiopaque mass. Additionally, a "coiled spring" appearance of the bowel may be visible due to the presence of a gallstone within the lumen, causing the bowel to twist and kink. The correct answer is not a diagnostic feature of gallstone ileus.

**Why Each Wrong Option is Incorrect**
* **Option A:** A large gallstone in the intestinal lumen is a characteristic feature of gallstone ileus and is often visible on a plain abdominal radiograph.
* **Option B:** The "coiled spring" appearance of the bowel is a classic radiographic sign of gallstone ileus due to the twisting and kinking of the bowel caused by the gallstone.
* **Option D:** A small bowel obstruction is a common complication of gallstone ileus, and the presence of air-fluid levels on a plain abdominal radiograph is a diagnostic feature of small bowel obstruction.

**Clinical Pearl / High-Yield Fact**
A key feature of gallstone ileus is the presence of a large gallstone in the intestinal lumen, which can cause a bowel obstruction. This condition is often associated with a history of gallstones and can be diagnosed on a plain abdominal radiograph.

**Correct Answer: D. A small bowel obstruction is not a diagnostic feature of gallstone ileus on a plain abdominal radiograph.**