Q. A 35-year-old male was admitted last night with a chief complaint of melena. The patient reports having dark, tarry stools for the past 3 days and experiencing moderate abdominal pain and fatigue. The patient has a history of Crohn's disease, which has been in remission for the past 5 years. He works as a banker and reports drinking 4 to 5 cups of coffee each day. He drinks alcohol daily and denies smoking tobacco. On physical examination, the patient is found to have a heart rate of 100/min, blood pressure of 140/80 mm Hg, and a temperature of 37.5 degrees Celsius. His abdomen is slightly distended and tender to the touch, with no masses or tenderness noted in the liver or spleen. A rectal examination reveals the presence of dark, tarry stool. Based on the information you have at this point in time, which of the following diagnostic procedures would be LEAST useful to identify the source of this patient’s bleeding?