**Core Concept**
Meckel's diverticulum is a congenital anomaly resulting from the incomplete closure of the vitelline duct, leading to the presence of ectopic gastric mucosa within the diverticulum. This ectopic tissue can cause complications such as bleeding, obstruction, or diverticulitis.

**Why the Correct Answer is Right**
Technetium-99m pertechnetate scan is the most sensitive and specific diagnostic test for Meckel's diverticulum with ectopic gastric mucosa. The scan takes advantage of the fact that the gastric mucosa is able to take up the radioactive material, allowing for visualization of the diverticulum. This occurs because the technetium-99m pertechnetate has a high affinity for mucoproteins found in the parietal cells of the stomach, which are present in the ectopic gastric mucosa.

**Why Each Wrong Option is Incorrect**
**Option A:** Barium enema is less sensitive and specific for Meckel's diverticulum, as it may not always show the diverticulum or the ectopic gastric mucosa.

**Option B:** CT scan or MRI can sometimes show the diverticulum but is not as sensitive or specific as a technetium-99m pertechnetate scan for detecting ectopic gastric mucosa.

**Option D:** Esophagogastroduodenoscopy (EGD) is not typically used for diagnosing Meckel's diverticulum with ectopic gastric mucosa, as it is more invasive and not as sensitive for this specific condition.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is a common congenital anomaly, occurring in approximately 2% of the population, and is often asymptomatic but can cause complications if the ectopic gastric mucosa is present.

**Correct Answer:** C. Technetium-99m pertechnetate scan.