Name: Sarah, 15 years. Chief Complaint: Pelvic pain and difficulty with menstruation. History: Sarah presents to the emergency department with severe lower abdominal pain that has been progressively worsening over the past few months. She also reports never having had a menstrual period. Her medical history is unremarkable, and there is no significant family history of gynecological issues. Clinical Examination: General Appearance: Sarah appears uncomfortable and distressed. Abdominal Examination: Lower abdominal tenderness without rebound or guarding. Genital Examination: On inspection, the external genitalia appear normal. However, a pelvic examination reveals a bulging bluish membrane obstructing the vaginal introitus. Investigations: Pelvic Ultrasound: Confirmatory imaging shows a fluid-filled mass in the pelvis. radiological exam image is attached 

What is the most likely cause of Sarah's symptoms?