A 30-year-old man presents with recurrent flushing, diarrhea, and weight loss. His examination is normal. Lab investigations reveal an elevated urinary 5-hydroxyindoleacetic acid. Which of the following is the most likely diagnosis?

Carcinoid syndrome is characterized by increased levels of 5-hydroxyindolacetic acid. The syndrome occurs in relation to malignant tumors that have metastasized, usually with hepatic implants. Gastrointestinal carcinoids are most commonly found in the appendix. These are very slow growing, thus the 5-year survival rate is 99%. Many carcinoids are discovered as incidental findings on autopsy.