A 25-year-old woman was hospitalized into the gynecological department with complaints of pain in her lower abdomen and high temperature of 39.7°C. Objectively, her blood pressure is 120/80 mm Hg, pulse- 108/min., of satisfactory rength and volume. The abdomen is moderately distended and sharply painful its lower segments. The Bloomberg's sign is positive in the hypogastrium. During vaginal examination, the uterus and its appendages cannot be palpated because of anterior abdominal wall rigidity. The posterior vaginal fornix is overhanging and sharply painful. What is the most likely daignosis in this case?