**Core Concept**
Postmenopausal bleeding (PMB) is any vaginal bleeding that occurs after the age of 50 years in a woman who has been postmenopausal for at least 6 months. The most common causes of PMB are atrophy, endometrial cancer, and cervical cancer.

**Why the Correct Answer is Right**
In a postmenopausal woman presenting with an episode of bleeding, the first step is to rule out malignancy. Endometrial cancer is a common cause of postmenopausal bleeding and is often asymptomatic in its early stages. Endometrial biopsy is a diagnostic procedure that involves taking a sample of tissue from the lining of the uterus (endometrium) to examine for cancer cells. This procedure is non-invasive and can be performed in an office setting.

**Why Each Wrong Option is Incorrect**
**Option A:** Assess for H/o HRT - While hormone replacement therapy (HRT) can cause bleeding in postmenopausal women, it is not the first step in evaluating PMB. A thorough history is important but should not delay further evaluation for malignancy.

**Option B:** Hysterectomy - This is a surgical procedure to remove the uterus and is not a diagnostic step in the evaluation of PMB.

**Option C:** PAPs Smear - While a PAP smear can detect cervical cancer, it is not the best initial step in evaluating PMB. Endometrial cancer is a more common cause of PMB in postmenopausal women.

**Option D:** DUB - Dysfunctional uterine bleeding (DUB) is a common cause of bleeding in premenopausal women but is less likely to be the cause of PMB.

**Clinical Pearl / High-Yield Fact**
Postmenopausal bleeding is a "red flag" symptom that requires immediate evaluation for malignancy. The American College of Obstetricians and Gynecologists (ACOG) recommends that postmenopausal women with bleeding undergo endometrial biopsy as the first step in evaluation.

**Correct Answer: D. Endometrial biopsy**