A collection of pus located between the peritoneum and the posterior abdominal wall is called a

• A. Retroperitoneal Abscess
• B. Lymphocele
• C. Mycotic Aneurysm
• D. Pseudoaneurysm

Answer: (A)

The key idea is that pus collected behind the peritoneum, between it and the back abdominal wall, forms a retroperitoneal abscess. The retroperitoneum sits behind the peritoneal lining and houses organs like the kidneys and pancreas; when infection produces purulent material in that space, the result is a retroperitoneal abscess. This differs from other fluid or vascular entities: a lymphocele is lymphatic fluid, usually after surgery and not primarily purulent; a mycotic aneurysm is an infected arterial dilation, a vascular wall abnormality rather than a pus collection; and a pseudoaneurysm is a contained arterial rupture with a blood-filled sac, also not an infectious fluid collection.