Hepatobiliary Disease USMLE Step 1 Practice Question

A 51-year-old woman presents with profuse watery diarrhea persisting during fasting. Vitals show BP 98/62, HR 112, RR 18, temp 37.2°C. She reports weakness and lightheadedness. Laboratory studies reveal serum potassium 2.8 mEq/L, pH 7.28, and markedly decreased gastric acid output on stimulation testing. CT abdomen shows a 2-cm enhancing lesion in the pancreatic tail. She denies recent antibiotic use. Which tumor most likely explains these findings?