**Core Concept**
The double bubble sign is a radiological finding that indicates an obstruction in the upper gastrointestinal tract, typically caused by a blockage in the duodenum. This sign is characterized by the presence of two distinct gas-filled structures (bubbles) in the upper abdomen, representing the stomach and the proximal duodenum.

**Why the Correct Answer is Right**
The double bubble sign is a classic radiographic feature of duodenal obstruction, which can be caused by conditions such as duodenal atresia or stenosis. In these cases, the obstruction prevents the normal passage of intestinal contents, resulting in the accumulation of gas in the stomach and the proximal duodenum. The double bubble sign is often visible on plain abdominal radiographs or ultrasound. Duodenal atresia and stenosis are congenital anomalies that can cause intestinal obstruction, and the double bubble sign is a crucial diagnostic clue.

**Why Each Wrong Option is Incorrect**
**Option A:** Duodenal atresia is a correct answer, but it's worth noting that volvulus can also cause a similar double bubble sign due to twisting of the duodenum.
**Option B:** Duodenal stenosis is also a correct answer, similar to duodenal atresia.
**Option C:** Volvulus can cause a double bubble sign due to twisting of the duodenum, but it's not the most common cause.
**Option D:** Intestinal perforation would typically cause free air under the diaphragm, rather than a double bubble sign.

**Clinical Pearl / High-Yield Fact**
The double bubble sign is a classic radiographic feature of duodenal obstruction, and it's essential to consider congenital anomalies such as duodenal atresia or stenosis in infants and young children presenting with vomiting and abdominal distension.

**Correct Answer: A. Duodenal atresia.