Image of the Week: Which neurotransmitter is the cause of these symptoms?

A 60-year-old woman presents with an 8-month history of colicky abdominal pain, diarrhea, and occasional flushing of the face and chest. Examination reveals bilateral wheezing and a distended, non-tender abdomen. Her levels of 5-hydroxyindoleacetic acid (5-HIAA) are elevated and a computed tomography scan demonstrates a hypervascular plaque-like mass in the terminal ileum with soft-tissue spokes radiating into the mesenteric fat. ...