**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. This condition often presents with gastrointestinal bleeding, obstruction, or diverticulitis.

**Why the Correct Answer is Right**
The correct answer is **Technetium-99m pertechnetate scan**. This imaging technique is based on the fact that the gastric mucosa in Meckel's diverticulum retains the ability to secrete pepsinogen, which is then labeled with Technetium-99m. This results in a high sensitivity and specificity for diagnosing Meckel's diverticulum, especially when it contains gastric or pancreatic tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is not the primary imaging modality for Meckel's diverticulum, as it may not be sensitive enough to detect the condition.
**Option B:** CT scan may show a diverticulum, but it is not as specific as a Technetium-99m pertechnetate scan for diagnosing Meckel's diverticulum.
**Option C:** Upper GI series may show a diverticulum, but it is not as sensitive as a Technetium-99m pertechnetate scan for detecting Meckel's diverticulum, especially if it is small or contains ectopic tissue.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is the most common congenital anomaly of the small intestine, occurring in approximately 2% of the population. It is often asymptomatic but can present with complications such as gastrointestinal bleeding, obstruction, or diverticulitis.

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