William Is A 25-year-old Who Comes To The Clinic With

William is a 25-year-old who comes to the clinic with a history of diarrhea, lower abdominal pain, and weight loss of approximately 40 pounds over the past 3 months. He also has noticed peripheral dependent edema. His hemoglobin is 8.0 and hematocrit 27%. What is the most likely cause of his signs and symptoms, and what diagnostic tests will he require?