Chris is a 20-year-old college student who has had abdominal pain for 3 days. it started at his umbilicus and was associated with nausea and vomiting. he was unable to find a comfortable position. yesterday, the pain became more severe and constant. now, he hesitates to walk, because any motion makes the pain much worse. it is localized just medial and inferior to his iliac crest on the right. which of the following is most likely?