**Core Concept**
The "double bubble" sign is a radiological finding typically associated with intestinal obstruction, particularly in the setting of duodenal or pyloric stenosis. It refers to the appearance of two air-filled structures (bubbles) in the upper abdomen on a plain abdominal X-ray.

**Why the Correct Answer is Right**
The double bubble sign is caused by the accumulation of air in the stomach and proximal duodenum, with the absence of air in the distal bowel coils. This occurs due to an obstruction at the level of the pylorus or the duodenum, preventing the normal passage of air and gastric contents into the small intestine. The resulting gas-filled stomach and proximal duodenum create the characteristic "double bubble" appearance on X-ray.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the double bubble sign is not typically associated with gastric outlet obstruction alone, which would more likely present with a single air-filled stomach rather than two distinct bubbles.

**Option B:** This option is incorrect because the double bubble sign is not characteristic of small bowel obstruction, which would more likely present with multiple air-filled loops of bowel and a lack of gas in the stomach and proximal duodenum.

**Option C:** This option is incorrect because the double bubble sign is not typically associated with ileus, which would more likely present with a lack of air in the bowel and a "gasless" abdomen on X-ray.

**Option D:** This option is incorrect because the double bubble sign is not characteristic of appendicitis, which would more likely present with right lower quadrant tenderness and a possible "cupola" sign on X-ray.

**Clinical Pearl / High-Yield Fact**
The double bubble sign is a classic radiological finding in pediatric patients with duodenal atresia or stenosis, and can be a useful diagnostic clue in the emergency setting.

**Correct Answer:** C.