Teenager with fever and abdominal pain

CT of retroperitoneal fasciitis
Axial (above), coronal (below left) and sagittal (below right) CT without contrast of the abdomen shows a large of amount of air in the right posterior pararenal space with associated fat stranding. The appendix was normal in size and there was an appendicolith that was outside of the appendix (not provided).

The diagnosis was right retroperitoneal fasciitis due to perforated appendicitis.