**Question:** The gold standard test for insulinoma

A. Ultrasound
B. CT scan
C. Endoscopic ultrasound
D. Magnetic resonance imaging (MRI)

**Correct Answer:** C. Endoscopic ultrasound

**Core Concept:** Insulinoma is a rare tumor of the pancreas, usually benign, that secretes excessive insulin, leading to hypoglycemia. The gold standard test for diagnosing insulinoma involves imaging techniques to localize the tumor and tests to assess hypoglycemia.

**Why the Correct Answer is Right:** Endoscopic ultrasound (EUS) is the preferred imaging test for insulinoma due to its high sensitivity and specificity in detecting pancreatic lesions, especially the small ones. EUS allows for detailed visualization of the pancreas, including the head, body, and tail, which is essential for accurate diagnosis. Additionally, it allows for tissue sampling (fine-needle aspiration biopsy) if a definitive diagnosis is required.

**Why Each Wrong Option is Incorrect:**

A. Ultrasound (US) is less accurate than EUS in detecting pancreatic lesions, particularly small tumors, and may miss the tumor due to its limited resolution.
B. Computed tomography (CT) scan is a useful imaging technique, but it has lower sensitivity and specificity compared to EUS for detecting pancreatic tumors, particularly small ones. CT scans may also fail to visualize the pancreatic head, which is crucial for insulinoma diagnosis.
D. Magnetic resonance imaging (MRI) is a valuable imaging modality but has lower sensitivity and specificity than EUS for detecting pancreatic tumors, particularly small ones. MRI is also less suitable for evaluating the pancreatic head, which is essential for insulinoma diagnosis.

**Clinical Pearl:** EUS is the gold standard test for diagnosing insulinoma. Its superiority lies in its ability to detect small tumors and visualize the entire pancreas, including the pancreatic head, which is vital for accurate insulinoma diagnosis. In cases where a definitive diagnosis is required, EUS can also perform fine-needle aspiration biopsy.