**Core Concept**
Insulinoma and sulfonylurea-induced hypoglycemia are two distinct clinical entities that require differentiation for proper management. The key to differentiation lies in understanding the pathophysiology of each condition. **Insulinomas are benign or malignant tumors of the pancreas that secrete excess insulin**, leading to hypoglycemia, whereas **sulfonylurea-induced hypoglycemia occurs due to the inhibition of sulfonylurea receptors on pancreatic beta cells**, causing an excessive release of insulin.

**Why the Correct Answer is Right**
The correct answer is **C. Glucose infusion test with 1-2 mg of glucagon**. This test is useful in differentiating between insulinoma and sulfonylurea-induced hypoglycemia. In insulinoma, the administration of glucagon will increase blood glucose levels, whereas in sulfonylurea-induced hypoglycemia, glucagon administration will have a minimal effect on blood glucose levels due to the inhibition of sulfonylurea receptors. This is because glucagon stimulates glycogenolysis and gluconeogenesis, which are impaired in insulinoma but not in sulfonylurea-induced hypoglycemia.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Fasting test** is not useful in differentiating between insulinoma and sulfonylurea-induced hypoglycemia, as both conditions can produce hypoglycemia during fasting.
* **Option B:** **Serum insulin levels** can be elevated in both insulinoma and sulfonylurea-induced hypoglycemia, making this test unreliable for differentiation.
* **Option D:** **Urine sulfonylurea screening** is not a reliable method for differentiating between insulinoma and sulfonylurea-induced hypoglycemia, as sulfonylurea metabolites can be present in the urine for an extended period.

**Clinical Pearl / High-Yield Fact**
In cases of suspected insulinoma, it is essential to perform a **72-hour fasting test** with serial measurements of blood glucose and insulin levels to confirm the diagnosis.

**Correct Answer: C. Glucose infusion test with 1-2 mg of glucagon**