You are answering a USMLE-style medical question. Question: A 57-year-old man comes to the physician because of a 3-week history of abdominal bloating and increased frequency of stools. He describes the stools as bulky, foul-smelling, and difficult to flush. He also has a 4-month history of recurrent dull upper abdominal pain that usually lasts for a few days, worsens after meals, and is not relieved with antacids. He has had a 10-kg (22-lb) weight loss in the past 4 months. He has no personal or family history of serious illness. He has smoked 1 pack of cigarettes daily for 37 years. He has a 12-year history of drinking 6 to 8 beers daily. He is 160 cm (5 ft 3 in) tall and weighs 52 kg (115 lb); BMI is 20 kg/m2. His vital signs are within normal limits. Abdominal examination shows mild epigastric tenderness without rebound or guarding. Bowel sounds are normal. The remainder of the examination shows no abnormalities. Which of the following is the most appropriate next step in diagnosis? Options: A) Abdominal CT scan B) Endoscopic ultrasonography C) Abdominal ultrasound D) Upper gastrointestinal endoscopy Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.