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?

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.