**Core Concept**
Meckel's diverticulum is a congenital anomaly of the small intestine, resulting from the incomplete closure of the vitelline duct. It is a true diverticulum containing all layers of the intestinal wall. A key diagnostic challenge is identifying the ectopic gastric mucosa within the diverticulum, which can cause gastrointestinal bleeding.

**Why the Correct Answer is Right**
Technetium-99m pertechnetate scan is the most sensitive investigation for diagnosing Meckel's diverticulum. This is because the ectopic gastric mucosa within the diverticulum selectively takes up the technetium-99m pertechnetate ion, allowing for its visualization. The mechanism of uptake is thought to be related to the presence of mucoproteins in the gastric mucosa, which bind to the technetium-99m ion.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography may be useful in detecting the diverticulum but is less sensitive than the technetium-99m scan for identifying the ectopic gastric mucosa.
**Option B:** Barium studies may show a filling defect or irregularity in the small intestine but are not as sensitive as the technetium-99m scan for diagnosing Meckel's diverticulum.
**Option C:** CT scan may be useful in detecting complications of Meckel's diverticulum, such as diverticulitis or bleeding, but is not the most sensitive investigation for diagnosing the diverticulum itself.

**Clinical Pearl / High-Yield Fact**
The technetium-99m pertechnetate scan is particularly useful in children, where the ectopic gastric mucosa is more likely to be present. A positive scan can also guide surgical intervention to remove the diverticulum.

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