**Core Concept**
Gastrointestinal stromal tumors (GISTs) are rare mesenchymal neoplasms that arise from the interstitial cells of Cajal or their precursors. The gold standard investigation for diagnosing and managing recurrent GISTs involves imaging techniques that provide detailed information about tumor size, location, and metastatic spread.

**Why the Correct Answer is Right**
The gold standard investigation for recurrent GISTs is imaging with **18F-FDG PET/CT**. This modality combines the metabolic information from **18F-FDG PET** with the anatomical details from **CT**. **18F-FDG PET** detects the increased glucose metabolism in GISTs, which is a hallmark of malignant cells. **CT** provides information about the size, location, and relationship of the tumor to surrounding structures. This combination allows for accurate detection and characterization of recurrent GISTs, guiding further management.

**Why Each Wrong Option is Incorrect**
**Option A:** Imaging with **MRI** is not the gold standard for GISTs, although it may be used as an adjunct to PET/CT for better soft tissue characterization.
**Option B:** **Endoscopy** is useful for evaluating the mucosal surface of the gastrointestinal tract but is not adequate for detecting and characterizing recurrent GISTs.
**Option C:** **Ultrasound** is not sensitive or specific enough for detecting and managing recurrent GISTs.

**Clinical Pearl / High-Yield Fact**
GISTs are the most common mesenchymal tumors of the gastrointestinal tract, and their diagnosis and management require a multidisciplinary approach, including imaging, surgery, and targeted therapy.

**Correct Answer:** C. Ultrasound