**Core Concept**
Meckel's diverticulum is a congenital anomaly of the small intestine, resulting from the incomplete closure of the vitelline duct. This condition can lead to various complications, including gastrointestinal bleeding, obstruction, and diverticulitis. The diagnosis of Meckel's diverticulum requires a combination of clinical suspicion and imaging studies.

**Why the Correct Answer is Right**
The best investigation to diagnose Meckel's diverticulum is a **Technetium-99m pertechnetate scan**, also known as a Meckel's scan. This nuclear medicine test takes advantage of the fact that the gastric mucosa, which is often present in Meckel's diverticulum, avidly takes up technetium-99m pertechnetate. The scan can detect the presence of ectopic gastric mucosa, which is a hallmark of Meckel's diverticulum. This test is particularly useful in pediatric patients, as it is non-invasive and can be performed quickly.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper GI series with barium. While an upper GI series can help identify the presence of a Meckel's diverticulum, it is not the most sensitive or specific test for diagnosing this condition.
**Option B:** CT scan. Although a CT scan can provide detailed images of the abdominal organs, it may not be able to detect the presence of ectopic gastric mucosa in Meckel's diverticulum.
**Option C:** Colonoscopy. A colonoscopy is primarily used to examine the large intestine and is not typically used to diagnose Meckel's diverticulum.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the presence of ectopic gastric mucosa in Meckel's diverticulum can lead to gastrointestinal bleeding, which is the most common presentation of this condition. This is due to the production of gastric acid by the ectopic gastric mucosa, which can cause ulceration and bleeding in the surrounding intestinal tissue.

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