**Core Concept**
Ectopic gastric mucosa of Meckel's diverticulum is a condition where gastric mucosa is found in a Meckel's diverticulum, a congenital anomaly of the small intestine. This ectopic tissue can cause complications such as gastrointestinal bleeding, obstruction, and diverticulitis. The presence of ectopic gastric mucosa is often diagnosed using the "rule of twos": the presence of a Meckel's diverticulum with a diameter of 2 cm or more, a location within 2 feet of the ileocecal junction, and a presence of ectopic gastric mucosa in 2% of the population.

**Why the Correct Answer is Right**
The ectopic gastric mucosa in Meckel's diverticulum is believed to arise from the failure of the vitelline duct to close during embryonic development. This leads to the presence of gastric mucosa in the small intestine, which can cause symptoms such as gastrointestinal bleeding, abdominal pain, and weight loss. The ectopic gastric mucosa can be identified using histopathological examination of the Meckel's diverticulum, which shows the presence of gastric mucosa, glands, and acid-producing cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely a distractor and does not provide any relevant information about the diagnosis of ectopic gastric mucosa of Meckel's diverticulum.

**Option B:** This option may refer to a different condition such as a gastrointestinal stromal tumor (GIST), which is a type of tumor that arises from the interstitial cells of Cajal in the gastrointestinal tract. However, GISTs are not associated with Meckel's diverticulum.

**Option C:** This option may refer to a condition such as Crohn's disease, which is a type of inflammatory bowel disease that can cause symptoms such as abdominal pain, diarrhea, and weight loss. However, Crohn's disease is not associated with Meckel's diverticulum or ectopic gastric mucosa.

**Option D:** This option is likely a distractor and does not provide any relevant information about the diagnosis of ectopic gastric mucosa of Meckel's diverticulum.

**Clinical Pearl / High-Yield Fact**
The "rule of twos" is a valuable clinical pearl that can help diagnose Meckel's diverticulum with ectopic gastric mucosa. This rule states that the presence of a Meckel's diverticulum with a diameter of 2 cm or more, a location within 2 feet of the ileocecal junction, and a presence of ectopic gastric mucosa in 2% of the population are all associated with an increased risk of complications.

**Correct Answer:** D.