**Core Concept**
Insulinoma is a rare tumor of the pancreas that produces excess insulin, leading to episodes of hypoglycemia (low blood sugar). Diagnosing insulinoma requires careful evaluation of various clinical and biochemical parameters.

**Why the Correct Answer is Right**
The D-xylose test is used to assess the intestinal absorption of carbohydrates, particularly in cases of malabsorption or celiac disease. It is not relevant to the diagnosis of insulinoma, which involves measuring insulin levels and their effects on glucose metabolism. Insulinoma diagnosis typically involves measuring fasting blood glucose levels, C-peptide levels, and the insulin/glucose ratio to confirm excessive insulin production.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fasting blood glucose levels are a crucial diagnostic criterion for insulinoma, as they help identify episodes of hypoglycemia.
* **Option B:** C-peptide levels are used to differentiate between endogenous insulin production (as in insulinoma) and exogenous insulin administration (as in insulin therapy).
* **Option C:** The insulin/glucose ratio is a sensitive indicator of insulinoma, as it helps identify periods of excessive insulin production relative to glucose levels.

**Clinical Pearl / High-Yield Fact**
When diagnosing insulinoma, it's essential to remember that patients often experience symptoms of hypoglycemia, such as confusion, tremors, and sweating, in addition to low blood glucose levels.

**✓ Correct Answer: D. D-xylose test. The D-xylose test is not used in the diagnosis of insulinoma.**