Ten days after Splenectomy for blunt abdominal trauma, a 23- year-old man complains of upper abdominal and lower chest pain exacerbated by deep breathing. He is anorectic but ambulatory and otherwise making satisfactory progress. On examination his temperature is38.20C and he has decreased breath sounds at left lung base. His abdominal wound appears to be healing well, bowel sounds are active and there are no peritoneal signs. Digital rectal examination is negative. WBC 12,500/mm3 with a shift to left. CXR show 'plate like' atelectasis of the left lung field. Abdominal radiograph shows a non-specific gas pattern in bowel and an air-fluid level in LUQ. Serum Amylase is 150 Somogyi units per dL. The most likely diagnosis is: