A 26-yeаr-оld femаle presents tо the Emergency Depаrtment with intense abdоminal pain for 6 hours, light-headedness, and a fainting episode that finally prompted her to seek medical attention. She has a history of gallstones and is concerned that she is having another gallbladder attack. She denies nausea and vomiting, and her last normal bowel movement was this morning. She reports that her LMP was 10 weeks ago. Vital signs: pulse 118; blood pressure, 86/68; respiratory rate, 20/min; oxygen saturation, 99%; and temperature, 37.3ºC orally. A β-human chorionic gonadotropin (β-hCG) blood test is pending. The NP performs an abdominal exam before her pelvic exam and, on palpation of her abdomen, finds involuntary rigidity and rebound tenderness. Which of the following is the most likely etiology for these assessment findings?