A 24-year-old woman has been hospitalized with complaints of fever of 39.2°C and pain in her lower abdominal segments. Objectively, pulse 102/min, blood pressure 120/70 mm Hg. The abdomen is moderately distended and sharply painful during palpation in the hypogastric region. During bimanual examination, the uterus and its appendages are not palpable because of tension of the anterior abdominal wall, the posterior fornix overhangs and is sharply painful. What is the most likely diagnosis in this case?