Q. A 35-year-old male is admitted with a history of epigastric pain, nausea and vomiting, and diarrhea. He has been experiencing these symptoms for the past 6 months, and they have been gradually increasing in frequency and intensity. The patient reports that he has lost 20 pounds in the past 6 months, despite eating more than usual. He also reports feeling tired and weak, and having trouble sleeping at night. He has no significant past medical history. On physical examination, the patient's vital signs are within normal limits, and there is no evidence of jaundice or dehydration. His cardiopulmonary assessment is unremarkable. You detect a tender, distended abdomen with a palpable mass in the epigastric region. You order an upper endoscopy, which reveals multiple ulcers in the duodenum, with a biopsy revealing hyperplasia of the gastrin-producing cells of the pancreas. You suspect a diagnosis of Zollinger-Ellison syndrome. Which of the following additional test results would confirm the diagnosis?