A26-year-old sports store manager comes to the clinic with severe right-sided abdominal pain for 12 hours. he began having a stomach ache yesterday with a decreased appetite, but today the pain seems to be just on the lower right side. he has had some nausea and vomiting but no constipation or diarrhea. his last bowel movement was last night and was normal. he has had no fever or chills. he denies any recent illnesses or injuries. his past medical history is unremarkable. he is engaged. he denies any tobacco or drug use and drinks four to six beers per week. his mother has breast cancer and his father has coronary artery disease. on examination he appears ill and is lying on his right side. his temperature is 38.8° and his heart rate is 170. his bowel sounds are decreased, and he has rebound and involuntary guarding at one third of the way between the anterior superior iliac spine and the umbilicus in the right lower quadrant. his rectal, inguinal, prostate, penile, and testicular examinations are normal. what is the most likely cause of his pain?