## **Core Concept**
The core concept being tested here is the diagnosis and follow-up of gastrointestinal stromal tumors (GISTs), specifically focusing on the gold standard investigation for detecting recurrent GIST. GISTs are the most common mesenchymal neoplasms of the gastrointestinal tract, and their management involves surgical resection and sometimes tyrosine kinase inhibitors.

## **Why the Correct Answer is Right**
The correct answer, **. PET/CT**, is considered the gold standard for detecting recurrent GIST because it combines the functional information of PET (Positron Emission Tomography) with the anatomical details of CT (Computed Tomography). PET/CT is particularly useful in GIST management due to the high metabolic activity of these tumors, which makes them easily detectable by PET. This modality is especially valuable in assessing early response to tyrosine kinase inhibitor therapy and in detecting recurrence.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . **CT abdomen** - While CT scans are commonly used for the initial diagnosis and follow-up of GISTs, they might not detect early recurrence or metabolic changes as effectively as PET/CT.
- **Option B:** . **MRI abdomen** - MRI can be used for assessing GISTs, especially for characterizing liver lesions or in cases where CT is not feasible. However, it is not as sensitive as PET/CT for detecting early recurrence or assessing treatment response.
- **Option D:** . **Endoscopy** - Endoscopy is useful for diagnosing mucosal lesions but is not suitable for detecting submucosal tumors like GISTs or their recurrence.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **PET/CT is particularly valuable in the context of GIST treatment response assessment**, especially with tyrosine kinase inhibitors. Early metabolic response seen on PET/CT can predict long-term outcome, making it an essential tool in the management of GIST.

## **Correct Answer:** . PET/CT