A 24-year-old nulliparous woman presents with a 3-month history of intermittent left iliac fossa pain and dyspareunia. Transvaginal ultrasound reveals a 7.2 × 6.8 cm well-circumscribed adnexal lesion. The structure marked **A** in the diagram shows a rounded hyperechoic mural nodule projecting into the cyst lumen with posterior acoustic shadowing, multiple linear echogenic interfaces representing hair, a fat-fluid level, and dense anterior acoustic shadowing obscuring the posterior wall. MRI confirms T1-hyperintense fat component suppressed on fat-saturated sequences. Tumor markers are normal. Which of the following is the most likely diagnosis and primary indication for surgical intervention in this patient?