**Core Concept**
Meckel's diverticulum is a congenital anomaly of the small intestine, resulting from the incomplete closure of the vitelline duct. The presence of ectopic mucosa in Meckel's diverticulum can lead to various complications, including gastrointestinal bleeding, obstruction, or diverticulitis. The diagnosis of ectopic mucosa in Meckel's diverticulum often relies on imaging studies and histopathological examination.

**Why the Correct Answer is Right**
Technetium-99m pertechnetate (Tc-99m) scan is a widely used imaging modality for diagnosing ectopic gastric mucosa in Meckel's diverticulum. The scan exploits the ability of Tc-99m to bind to the mucosa of the stomach and other ectopic gastric tissue, leading to a high uptake of the radiotracer in affected areas. This allows for the visualization of Meckel's diverticulum containing ectopic gastric mucosa, making it a valuable diagnostic tool.

**Why Each Wrong Option is Incorrect**
**Option A:** Barium studies are not specific for diagnosing ectopic mucosa in Meckel's diverticulum, as they may not always demonstrate the presence of ectopic tissue.
**Option B:** Endoscopy is not typically used for diagnosing Meckel's diverticulum, as it is often located in the small intestine, making it difficult to access.
**Option C:** Ultrasound may be used to identify Meckel's diverticulum, but it is not the most sensitive or specific method for detecting ectopic mucosa.

**Clinical Pearl / High-Yield Fact**
It is essential to consider Meckel's diverticulum in the differential diagnosis of gastrointestinal bleeding, especially in children and young adults, as it can be a life-threatening condition if left untreated.

**Correct Answer: C. Ultrasound.