Adenomyosis

A 48-year-old multiparous woman presents with a 2-year history of progressive secondary dysmenorrhea, menorrhagia, and chronic pelvic pain. On bimanual examination, the uterus is tender, globular, and uniformly enlarged to approximately 12 weeks size with a soft, boggy consistency. Transvaginal ultrasound shows a diffusely thickened, trabeculated myometrium with small cystic foci and heterogeneous echotexture, as marked **A** in the diagram. MRI confirms junctional zone thickness of 14 mm with a junctional-to-myometrial ratio of 45%. Histopathology from a recent biopsy demonstrates endometrial glands and stroma within the myometrium accompanied by reactive smooth muscle hyperplasia. Which of the following best explains the gross and histological appearance of the structure marked **A**?