Review Question - QID 213655

A 65-year-old previously healthy man presents to his primary care provider with complaints of abdominal cramping and diarrhea. He states that he has had these symptoms for the past 3 weeks and presents at this time because he believes the symptoms are becoming more frequent. He has not had any recent travel or sick contacts, and does not report any fevers. However, he reports that in the last week he has noticed episodes of warmth and facial flushing, which he is self-conscious about. Of note, the patient has lost approximately 10 pounds in the last 6 months. On exam, the patient’s temperature is 98.2°F (36.8°C), blood pressure is 126/78 mmHg, pulse is 70/min, and respirations are 12/min. Lungs are clear to auscultation, and cardiac exam demonstrates normal heart sounds with a regular rate. The patient does not have any acute abdominal tenderness to palpation but reports vague diffuse abdominal cramping. Stool is negative for blood. The patient is asked to perform a 24-hour urine collection, which demonstrates elevated 5-hydroxyindoleacetic acid. This patient's disease can result in symptoms similar to a deficiency in which of the following vitamins?