## **Core Concept**
Insulinoma is a tumor of the pancreas that produces excess insulin, leading to hypoglycemia. The diagnosis of insulinoma involves demonstrating the presence of hypoglycemia with an inappropriately elevated insulin level. The key laboratory finding that confirms the suspicion of insulinoma is the presence of hypoglycemia with elevated insulin and C-peptide levels.

## **Why the Correct Answer is Right**
The correct answer, **C. Elevated insulin and C-peptide levels**, confirms the suspicion of insulinoma because it indicates that the hypoglycemia is due to endogenous insulin production rather than an exogenous source. Insulinoma produces both insulin and C-peptide (a byproduct of proinsulin cleavage), whereas exogenous insulin administration does not produce C-peptide. Therefore, finding elevated levels of both insulin and C-peptide during an episode of hypoglycemia supports the diagnosis of insulinoma.

## **Why Each Wrong Option is Incorrect**
- **Option A: Low insulin and C-peptide levels**. This would suggest that the hypoglycemia is not due to excess insulin, making insulinoma unlikely.
- **Option B: Elevated insulin but low C-peptide levels**. This profile is consistent with exogenous insulin administration, not insulinoma, as C-peptide is not produced when exogenous insulin is given.
- **Option D: Low insulin and elevated C-peptide levels**. This combination does not support the diagnosis of insulinoma since insulin levels are low.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for diagnosing insulinoma is the "72-hour fasting test," which is considered the gold standard for diagnosis. During this test, patients fast under close supervision, and blood glucose, insulin, and C-peptide levels are monitored. The presence of hypoglycemia with inappropriately elevated insulin and C-peptide levels confirms the diagnosis.

## **Correct Answer: C. Elevated insulin and C-peptide levels**