**Question:** Which of the following tests is not used in the diagnosis of insulinoma -

A. Plasma glucose measurement
B. Plasma insulin measurement
C. Plasma C-peptide measurement
D. Imaging studies (e.g., CT, MRI)

**Core Concept:**
Insulinoma is a type of endocrine tumor, usually benign, that originates from the beta cells of the pancreatic islets of Langerhans. The correct diagnosis of insulinoma involves a combination of clinical evaluation, laboratory tests, and imaging studies.

**Why the Correct Answer is Right:**
The correct answer is **D. Imaging studies (e.g., CT, MRI)** because these methods are used to identify the location of the insulinoma for surgical excision or further evaluation. They help visualize the tumor and its size, shape, and relationship to surrounding structures.

**Why Each Wrong Option is Incorrect:**

A. **Plasma glucose measurement (Plasma glucose testing)** is a useful test for evaluating hypoglycemia, but it is not specific for insulinoma diagnosis. Hypoglycemia can be caused by various conditions, and measuring blood glucose alone is not sufficient for diagnosing insulinoma.

B. **Plasma insulin measurement** is also useful in evaluating hypoglycemia. However, insulin levels are influenced by factors other than insulinoma, such as stress, fasting, and glucagon administration. This test alone is not sufficient for diagnosing insulinoma.

C. **Plasma C-peptide measurement** is also relevant in evaluating hypoglycemia, but similarly to insulin, C-peptide levels can be influenced by various factors, making it insufficient as a standalone test for insulinoma diagnosis.

**Clinical Pearl:**
Insulinoma diagnosis typically involves a combination of clinical evaluation, laboratory tests (e.g., glucose and insulin/C-peptide testing during fasting and glucose tolerance tests), and imaging studies (e.g., ultrasound, CT, MRI, or angiography) to confirm the presence of a functional pancreatic beta-cell tumor causing hypoglycemia.