This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Trauma, Subject name: Surgery
Nine days following splenectomy, a 13-year- old patient presents with fever and leukocytosis. The chest x-ray shows free air under the diaphragm. What is the most likely diagnosis? 
Select and explain. 
A. Left lower lobe pneumonia, B. Postsplenectomy sepsis, C. Gastric wall ulcer, D. Left colon perforation
D: Left colon perforation
Answer: D: Left colon perforation 
Explanation: Colon perforation is likely to show free air under the left hemidiaphragm. A subphrenic abscess presents with fever, leukocytosis, and a left pleural effusion. Gastric wall necrosis may likewise result in perforation with free air. There is air below the diaphragm following laparotomy, but it usually manifests symptoms clearly within the first week after operation.