Acute appendicitis, a disease found predominantly in adolescents and young adults, is characterized histologically by acute inflammatory cells (neutrophils) within the mucosa and muscular wall. Clinically, acute appendicitis causes right lower quadrant pain, nausea, vomiting, a mild fever, and a leukocytosis in the peripheral blood. These symptoms may not occur in the very young or the elderly. The inflamed appendiceal wall may become gangrenous and perforate in 24 to 48 h. Even with classic symptoms, the appendix may be histologically unremarkable in up to 20% of the cases. False-positive diagnoses are to be preferred to the possible severe or fatal complications of a false-negative diagnosis of acute appendicitis that results in rupture. Lymphoid hyperplasia with multinucleated giant cells (Wahin-Finkeldey giant cells) is characteristic of measles (rubeola). These changes can be found in the appendix, but this is quite rare. Dilation of the lumen of the appendix, called a mucocele, may be caused by mucosal hyperplasia, a benign cystadenoma, or a malignant mucinous cystadenocarcinoma. If the latter tumor ruptures, it may seed the entire peritoneal cavity, causing the condition called pseudomyxoma peritonei. The most common tumor of the appendix is the carcinoid tumor. Grossly it is yellow in color and is typically located at the tip of the appendix. Histologically, carcinoids are composed of nests or islands of monotonous cells. Appendiceal carcinoids rarely metastasize Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition