Review Question - QID 109492

A 72-year-old man presents to his primary care physician with the symptom of generalized malaise over the last month. He also has abdominal pain that has been persistent and not relieved by ibuprofen. He has unintentionaly lost 22 pounds recently. During this time, the patient has experienced intermittent diarrhea when he eats large meals. The patient has a past medical history of alcohol use, obesity, diabetes mellitus, hypertension, IV drug use, and asthma. His current medications include disulfiram, metformin, insulin, atorvastatin, lisinopril, albuterol, and an inhaled corticosteroid. The patient attends weekly Alcoholics Anonymous meetings and was recently given his two week chip for not drinking. His temperature is 99.5°F (37.5°C), blood pressure is 100/57 mmHg, pulse is 88/min, respirations are 11/min, and oxygen saturation is 98% on room air. The patient’s abdomen is tender to palpation, and the liver edge is palpable 2 cm inferior to the rib cage. Neurologic exam demonstrates gait that is not steady. Which of the following is the best initial diagnostic test for this patient?