**Core Concept**
The "double bubble sign" is a radiological finding seen in infants and children, typically on an abdominal X-ray or ultrasound. It is indicative of intestinal obstruction, specifically duodenal atresia, where there is a congenital blockage of the duodenum, the first portion of the small intestine.

**Why the Correct Answer is Right**
The double bubble sign is caused by the accumulation of gas in the stomach and the proximal duodenum, resulting in two distinct air-filled structures visible on imaging. This is due to the obstruction of the duodenum, which prevents the normal flow of intestinal contents and gas. The duodenum is usually connected to the stomach via the pylorus, and the obstruction prevents the normal passage of gas and food through the duodenum, resulting in the characteristic double bubble appearance. Duodenal atresia is often associated with other congenital anomalies, such as Down syndrome or cardiac defects.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the double bubble sign, which is a specific radiological finding associated with duodenal atresia. Other types of intestinal obstruction may have different radiological findings.
* **Option B:** This option is incorrect because the double bubble sign is a specific finding associated with duodenal atresia, and not with gastric outlet obstruction. Gastric outlet obstruction may result in a different radiological appearance, such as a "string sign" or a "bird's beak" appearance.
* **Option C:** This option is incorrect because the double bubble sign is not typically associated with ileal atresia. Ileal atresia may result in a different radiological appearance, such as a "ground glass" appearance or a "string sign".
* **Option D:** This option is incorrect because the double bubble sign is a specific finding associated with duodenal atresia, and not with pyloric stenosis. Pyloric stenosis may result in a different radiological appearance, such as a "olive" or a "string sign".

**Clinical Pearl / High-Yield Fact**
The double bubble sign is a classic radiological finding associated with duodenal atresia, and it is often seen in infants and children with congenital heart disease or Down syndrome. It is essential to recognize this sign to diagnose duodenal atresia promptly and initiate appropriate treatment.

**Correct Answer: B. Pyloric stenosis.**