A 61 year old man undergoes CT scan study of the abdomen for unexplained abdominal distension. Low attenuation intraperitoneal collections with enhancing septae are demonstrated. There is scalloping of the liver border and omental thickening. Which one of the following is most likely to be the underlying cause?

**Core Concept**
The patient's imaging findings are suggestive of a complex peritoneal process, likely a malignancy, given the presence of enhancing septae and scalloping of the liver border. The clinical presentation of abdominal distension and the imaging findings are characteristic of a specific peritoneal process.

**Why the Correct Answer is Right**
The presence of low attenuation intraperitoneal collections with enhancing septae is suggestive of a peritoneal carcinomatosis or a pseudomyxoma peritonei. The enhancing septae represent areas of neoplastic or tumor tissue, which are highly vascularized and thus enhance with contrast. The scalloping of the liver border is a result of the mass effect exerted by the peritoneal process, which causes the liver to be displaced and its border to be scalloped. Omental thickening is also consistent with peritoneal carcinomatosis or pseudomyxoma peritonei.

**Why Each Wrong Option is Incorrect**
**Option A:** While a perforated viscus can cause peritonitis and potentially lead to adhesions and thickening of the omentum, it would not typically present with enhancing septae on imaging.
**Option B:** A liver abscess or pyogenic liver infection would not typically cause scalloping of the liver border or omental thickening.
**Option C:** A pancreatic pseudocyst can cause abdominal distension and potentially cause adhesions and thickening of the omentum, but would not typically present with enhancing septae on imaging.

**Clinical Pearl / High-Yield Fact**
Pseudomyxoma peritonei and peritoneal carcinomatosis can present with a similar imaging appearance, but pseudomyxoma peritonei typically arises from a mucinous tumor of the appendix, whereas peritoneal carcinomatosis is a result of metastatic disease from a primary cancer.

**Correct Answer: C. Peritoneal carcinomatosis**

✓ Correct Answer: B. Cystadenocarcinoma of the appendix