**Core Concept**
Meckel's diverticulum is a congenital anomaly of the small intestine, resulting from the incomplete closure of the vitelline duct. Ectopic mucosa within the diverticulum can cause various complications, including gastrointestinal bleeding, obstruction, and inflammation.

**Why the Correct Answer is Right**
The diagnosis of ectopic mucosa in Meckel's diverticulum is primarily based on the presence of gastric or pancreatic mucosa within the diverticulum. This can be confirmed through **histopathological examination** of a biopsy specimen obtained during endoscopy or surgery. The presence of ectopic mucosa is often associated with the presence of **gastrin-secreting cells**, which can lead to gastric acid secretion and subsequent bleeding.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely a distractor and does not provide a valid diagnostic method for Meckel's diverticulum.
* **Option B:** While imaging studies such as ultrasound, CT, or MRI can help identify Meckel's diverticulum, they do not specifically diagnose ectopic mucosa.
* **Option D:** This option is incorrect as Meckel's scan (technetium-99m pertechnetate scan) is primarily used to identify the presence of gastric mucosa within the diverticulum, but it is not the only diagnostic method.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is a common congenital anomaly, occurring in approximately 2% of the population. The presence of ectopic mucosa within the diverticulum can lead to significant complications, making early diagnosis and treatment crucial.

**Correct Answer: A. Histopathological examination**