**Core Concept**
Meckel's diverticulum is a congenital anomaly of the small intestine, resulting from the incomplete closure of the vitelline duct. It is characterized by a true diverticulum, containing ectopic gastric or pancreatic tissue, which can cause complications such as bleeding, obstruction, or inflammation.

**Why the Correct Answer is Right**
The best investigation to diagnose Meckel's diverticulum is a **Technetium-99m pertechnetate (99mTc) scan**, also known as a Meckel's scan. This nuclear medicine test takes advantage of the fact that the ectopic gastric mucosa in Meckel's diverticulum can bind to the radioactive tracer, leading to a localized accumulation of the isotope. The scan is highly sensitive and specific for Meckel's diverticulum, especially when the diverticulum contains gastric or pancreatic tissue.

**Why Each Wrong Option is Incorrect**
* **Option A:** Upper GI endoscopy is not the best investigation for Meckel's diverticulum, as it may not visualize the diverticulum, especially if it is located in the small intestine.
* **Option B:** CT scan or MRI may be useful in evaluating complications of Meckel's diverticulum, such as inflammation or obstruction, but they are not the primary diagnostic tool for the condition.
* **Option D:** Colonoscopy is not relevant for diagnosing Meckel's diverticulum, which is a small intestinal anomaly.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is the most common congenital anomaly of the small intestine, with an incidence of approximately 2%. It is often asymptomatic but can cause significant complications, especially if it contains ectopic gastric mucosa that secretes acid.

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