J.K. is a 52-year-old state auditor who presents to his personal physician with a chief complaint of muscle weakness and dizziness. He has lost 15 pounds in the past 5 months despite a regular appetite and exercise regimen (diminished due to the weakness). On examination, the doctor notices benign freckles formed around the lips and low blood sugar. Name and describe a diagnosis that fits this case. Name and describe a diagnostic test that confirms your theory.