**Core Concept**
Rigler's triad is a radiological finding associated with a specific condition. It involves the combination of **small bowel obstruction**, **pneumobilia**, and **an abdominal mass**. This triad is classically seen in gallstone ileus.

**Why the Correct Answer is Right**
The correct answer is related to gallstone ileus, a condition where a gallstone enters the bowel through a fistula, most commonly a cholecystoenteric fistula, and causes a mechanical obstruction. The **gallstone** acts as a foreign body, obstructing the **small intestine**, while **pneumobilia** is seen due to the fistulous connection between the gallbladder and the intestine, allowing air to enter the biliary tree. An **abdominal mass** may be palpable if the obstruction is significant or if there is a large stone.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not specifically relate to the combination of findings in Rigler's triad.
**Option B:** Similarly, this option does not accurately describe the condition associated with Rigler's triad.
**Option D:** This choice is also incorrect as it does not match the classic description of Rigler's triad.

**Clinical Pearl / High-Yield Fact**
Rigler's triad is a significant finding because it suggests a gallstone ileus, which is a surgical emergency. Recognizing this triad on imaging can prompt urgent intervention, highlighting the importance of familiarity with this radiological sign.

**Correct Answer:** D. Gallstone ileus.