Usually presents as abdominal pain, classically periumbilical crampy pain that becomes localized to the right lower quadrant associated with anorexia nausea and vomiting, followed by fever.  The abdominal exam demonstrates localized right lower quadrant tenderness with guarding(muscle tightness due to adjacent peritoneal irritation from infection in the outer wall of the appendix).  Gastroenteritis generally does not cause guarding since the infection is on the inside of the intestines.  As pressure builds inside the obstructed infected appendix perforation occurs leading to more diffuse tenderness.  The diagnosis is helped by an elevated WBC count but a normal WBC count does not exclude appendicitis.  A right lower quadrant ultrasound may demonstrate a thickened appendix but since the test is operator dependent not visualizing the appendix does not exclude the diagnosis.  A CT scan is over 95% diagnostic and generally used an inconclusive cases but since there is a risk to radiation exposure this test is increasingly avoided if the diagnosis for a surgical condition is established, since the patient can have a laparoscopy which will confirm the diagnosis and treat the problem.  Laparoscopy has been associated with decreased wound infections compared with open surgery.  Nonoperative treatment with antibiotic therapy alone for nonperforated appendicitis has a significant failure rate.