A 28-year-old woman presents with acute onset right lower quadrant pain for 18 hours. On examination, she is afebrile, and there is localized tenderness at McBurney's point without rebound or guarding. WBC is 11,200/μL. Abdominal ultrasound shows an appendix measuring 6.5 mm with mild periappendiceal echogenicity but no free fluid. The sonographer notes the appendix is compressible. What is the most likely diagnosis, and what is the most appropriate next step?