**Core Concept**
The "double bubble sign" is a radiological finding seen on abdominal X-rays or ultrasound, which is indicative of intestinal obstruction in neonates. It is characterized by the presence of two gas-filled structures (bubbles) in the upper abdomen, typically representing the duodenum and the stomach.

**Why the Correct Answer is Right**
The double bubble sign occurs due to the obstruction of the small intestine, usually at the level of the duodenum, which prevents the normal passage of intestinal contents. This leads to the accumulation of gas in the stomach and the proximal duodenum, resulting in the characteristic double bubble appearance on imaging studies. The most common cause of this sign is duodenal atresia or stenosis, which is a congenital anomaly that involves the narrowing or complete blockage of the duodenum.

**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, which would more likely result in a single gas-filled structure in the upper abdomen (the stomach).

**Option B:** This option is incorrect because the "double bubble sign" is not seen in cases of intestinal perforation, which would result in free air under the diaphragm on an upright chest X-ray.

**Option C:** This option is incorrect because the "double bubble sign" is not a characteristic finding in cases of neonatal appendicitis, which would more likely result in a mass or inflammation in the right lower quadrant of the abdomen.

**Option D:** This option is incorrect because the "double bubble sign" is not seen in cases of pyloric stenosis, which would result in a single gas-filled structure in the upper abdomen (the stomach) and a visible pyloric mass on imaging studies.

**Clinical Pearl / High-Yield Fact**
The double bubble sign is a classic radiological finding in neonates with duodenal atresia or stenosis, and its presence should prompt immediate evaluation for this condition, including abdominal X-rays, ultrasound, or upper GI studies.

**Correct Answer: D. Pyloric stenosis**