The mother of a 5-year-old male tells you that he was vomiting earlier today and complained of moderate pain around his umbilicus. She tells you that just before she called, the pain worsened, and he now has severe pain. When you ask him where the pain is the worst, he points to the right lower abdominal quadrant. His vital signs are P 98, R 26, BP 96/64, and SpO2 is 98% on room air. You should suspect: