A 24-year-old female presents to an urgent care clinic complaining of abdominal pain that started approximately 18 hours ago. She describes the pain as initially vague and periumbilical, but it has since shifted and intensified in her right lower quadrant. She reports feeling nauseous, had one episode of non-bilious emesis, and has not eaten since yesterday evening due to anorexia. She denies dysuria, vaginal discharge, or changes in bowel habits. Her last menstrual period was 2 weeks ago. On examination, her temperature is 38.5 C (101.3 F), blood pressure is 115/70 mm Hg, pulse is 98/min, and respirations are 18/min. She appears uncomfortable and prefers to lie still on the examination table. Her oral mucosa is moist. Abdominal examination reveals normal contour with active bowel sounds. There is exquisite tenderness with voluntary guarding and rebound tenderness at McBurney's point. Rovsing's sign is positive. Psoas and Obturator signs elicit pain. A pelvic examination is unremarkable. Based on her presentation and initial findings, what is the most likely diagnosis and the immediate definitive management?