A 30-year-old man presents to the clinic with recurrent symptoms of flushing, diarrhea, and weight loss. He cannot associate the symptoms with any particular activity, time of day or food ingestion. His past medical history is negative and he is not taking any prescription or recreational medications.On physical examination, his blood pressure is 126/74 mmHg, and the heart rate is 72/min and regular. His remaining physical examination is completely normal. Lab investigations reveal an elevated urinary 5-hydroxyindoleacetic acid (5-HIAA) Which of the following is the most likely diagnosis?