A 43-year-old male was admitted with fever, abdominal distension, and tenesmus for 2 weeks. Physical examination revealed abdominal distension without abdominal wall varicose veins, doughy abdomen, mild tenderness and rebound tenderness. Ascitic fluid analysis showed bloody exudate with no pathological cells. Stool examination showed 5-8 red blood cells (RBC)/HP and 2-4 white blood cells (WBC)/HP. Which of the following tests should be performed first to confirm the diagnosis?