**Core Concept**
Dysfunctional uterine bleeding (DUB) is a common cause of abnormal uterine bleeding in premenopausal women, characterized by irregular or prolonged menstrual bleeding without any identifiable cause. In this case, the patient's endometrial thickness of 8 mm on transvaginal ultrasound is a critical finding.

**Why the Correct Answer is Right**
The next step in the management of DUB with an endometrial thickness of 8 mm is to perform an endometrial biopsy to rule out endometrial hyperplasia or cancer. This is because endometrial thickness is a risk factor for endometrial cancer, and an endometrial biopsy can provide a definitive diagnosis. The biopsy involves taking a sample of tissue from the endometrium, which is then examined for any abnormal cell growth.

**Why Each Wrong Option is Incorrect**
**Option A:** Medical management with hormonal therapy may be considered for DUB, but it is not the next step in this patient with an abnormal endometrial thickness.
**Option B:** Hysteroscopy may be performed to evaluate the uterine cavity, but it is not the initial step in managing DUB with an endometrial thickness of 8 mm.
**Option C:** Further imaging studies such as MRI may be used to evaluate the endometrium, but they are not necessary before performing an endometrial biopsy.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends performing endometrial biopsy on women with postmenopausal bleeding or an endometrial thickness of 5 mm or greater.

**Correct Answer:** D. Endometrial biopsy.