This patient with abdominal pain and chalky white mesenteric lesions likely had acute pancreatitis.  To prevent autodigestion, most pancreatic digestive enzymes (except for amylase and lipase) are secreted in an inactive form and are activated on arrival in the duodenum.  In acute pancreatitis, intrapancreatic activation of enzymes result in acinar cell damage and pancreatic inflammation.

The release of lipase and other digestive enzymes from the inflamed pancreas damages nearby adipose cells.  Liberated fatty acids bind calcium ions, precipitating as insoluble calcium salts (ie, saponification) that have a chalky white appearance grossly.  Microscopically, necrotic adipocytes (ie, anucleate adipocytes with necrotic debris) and blue calcium deposits are seen (compared to a normal pancreas).

In mild cases, small areas of fat necrosis may be found within and around the pancreas, along with an edematous and inflamed pancreatic parenchyma.  If the inflammatory process continues, acute necrotic pancreatitis develops.  This is characterized by parenchymal necrosis and more extensive fat necrosis, which can spread to involve the mesentery, omentum, and other parts of the abdominal cavity (due to widespread lipase activity).  Elastase-mediated destruction of blood vessel walls can cause hemorrhage into the necrotic areas (ie, hemorrhagic pancreatitis), and black hemorrhage may be seen grossly.

(Choice B)  Bacterial peritonitis is characterized by a dull-appearing peritoneal surface with areas of viscous white-yellow suppurative exudate.  Microscopic examination of the ascitic fluid shows numerous neutrophils.

(Choice C)  In celiac disease, gross examination of an involved bowel segment would show normal-appearing serosa and mesentery.

(Choice D)  Crohn disease often shows mesenteric fat grossly extending around the serosa of an affected intestinal segment (ie, "creeping fat").  The serosa looks dull-gray, edematous, and granular.  The mesentery of an involved segment may become fibrotic due to chronic inflammation.

(Choice E)  With intestinal perforation, a full-thickness defect in the intestinal wall is seen on gross examination.

(Choice F)  With mesenteric ischemia, gross examination of an affected bowel segment would show dusky red serosa, edema, and hemorrhagic mucosa.  Microscopically, areas of coagulative necrosis may be present.

Educational objective:
In acute pancreatitis, release of lipase and other digestive enzymes causes fat necrosis with precipitation of insoluble calcium salts (ie, saponification), imparting a chalky white gross appearance.  Microscopically, necrotic fat cells with calcium deposits are seen.  In severe cases, fat necrosis can involve the mesentery, omentum, and other parts of the abdominal cavity.