A 45-year-old woman undergoes a supervised 72-hour fast for evaluation of recurrent fasting hypoglycemia. At hour 52, glucose is 42 mg/dL and simultaneous labs show insulin 14 µU/mL, C-peptide 3.2 ng/mL, and beta-hydroxybutyrate 0.4 mmol/L. Which diagnosis is confirmed?
- A Exogenous insulin administration
- B Insulinoma ✓
- C Factitious sulfonylurea use
- D Reactive (postprandial) hypoglycemia
Correct answer: B. Insulinoma
Explanation
During a fast, insulin of at least 3 µU/mL with glucose below 45 mg/dL indicates inappropriate insulin secretion. Elevated C-peptide confirms endogenous (not exogenous) insulin. Suppressed beta-hydroxybutyrate reflects insulin-mediated suppression of lipolysis, consistent with insulinoma. Factitious insulin use would show high insulin but low C-peptide.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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