**Core Concept**
The patient's symptoms, particularly the enlargement of the uterus without nodularity and the extension of endometrial glands to the myometrium, suggest a condition where there is abnormal growth of endometrial tissue outside the uterus.

**Why the Correct Answer is Right**
The correct diagnosis is adenomyosis, a condition where endometrial tissue grows into the myometrium, the smooth muscle layer of the uterus. This leads to the symptoms of uterine enlargement and pelvic discomfort. The absence of nodularity on pelvic examination rules out conditions like uterine fibroids or other types of tumors. The biopsy findings confirming the extension of endometrial glands into the myometrium support the diagnosis of adenomyosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Endometriosis is a condition where endometrial tissue grows outside the uterus, but it typically presents with nodularity and pelvic pain, not just uterine enlargement.
**Option B:** Uterine fibroids are benign tumors of the uterus that can cause uterine enlargement, but they do not involve the extension of endometrial glands into the myometrium.
**Option C:** Leiomyoma is another term for uterine fibroids, which are not characterized by the extension of endometrial glands into the myometrium.
**Option D:** Endometrial carcinoma is a type of cancer that originates from the endometrium, but it typically presents with postmenopausal bleeding, pelvic mass, or other symptoms not described in this patient.

**Clinical Pearl / High-Yield Fact**
Adenomyosis can be challenging to diagnose, and a definitive diagnosis often requires histopathological examination of the uterus. It is essential to differentiate adenomyosis from other conditions that cause uterine enlargement, such as uterine fibroids or endometrial cancer.

**Correct Answer:** D.