**Core Concept:**
The double bubble sign is a clinical sign observed in gastrointestinal (GI) tract disorders, particularly in conditions affecting the terminal ileum and the rectum. It is characterized by the presence of two gas-filled structures, resembling a double bubble or a "double-barreled gun," seen on a plain abdominal X-ray.

**Why the Correct Answer is Right:**
The correct answer, **D**, refers to the condition known as Meckel's diverticulum. Meckel's diverticulum is a remnant of the midgut remnant that persists after the development of the normal gastrointestinal tract. In this condition, the double bubble sign is due to the presence of two gas-filled structures - one representing the air-filled small intestine and the other representing the air-filled rectum.

**Why Each Wrong Option is Incorrect:**

A. This option refers to an air-filled colon, which is typically not associated with the double bubble sign. The double bubble sign is specific to conditions involving the small intestine and rectum.

B. This option refers to a gas-filled bladder, which is unrelated to the double bubble sign in the context of GI tract disorders.

C. This option refers to a gas-filled sigmoid colon, which is not the primary focus of the double bubble sign. The double bubble sign is more likely to occur due to small bowel abnormalities like Meckel's diverticulum or Hirschsprung's disease.

**Clinical Pearl / High-Yield Fact:**
In clinical practice, the double bubble sign can be a crucial clue for diagnosing conditions like Meckel's diverticulum or Hirschsprung's disease, as these conditions often present with the double bubble sign and require appropriate clinical suspicion and diagnostic workup for definitive diagnosis and management.