Step 1: Orient the slice: the endometrial lining sits at the top and the serosa at the bottom. The key finding is islands of lining-type tissue burrowing down into the muscular wall between those two surfaces.
Step 2: Tissue from the womb lining sitting WITHIN the myometrium is the definition of adenomyosis. The patient's painful periods, painful intercourse and persistent pelvic pain are the classic clinical correlates.
Step 3: Adenomyosis comes in diffuse and focal forms, and because medical therapy is often inadequate, hysterectomy is the ultimate cure - explaining why this whole-uterus specimen exists.
Step 4: Separate it from the look-alikes: a leiomyoma (the same lesion as a 'fibroid') is a sharply demarcated whorled muscle nodule, not glandular tissue diffusing through muscle; endometriosis places identical tissue OUTSIDE the uterus altogether. Neither matches intramyometrial glands.
\[\boxed{\text{Adenomyosis}}\]