## **Core Concept**
Postmenopausal bleeding (PMB) is a significant symptom that warrants thorough investigation due to its association with potential malignancies and other gynecological conditions. The primary goal is to rule out cancer and determine the cause of bleeding.

## **Why the Correct Answer is Right**
In a postmenopausal woman presenting with vaginal bleeding, an **endometrial biopsy** or **transvaginal ultrasound** to assess endometrial thickness is crucial as the initial step. However, given the provided options and focusing on immediate action, **D. Endometrial biopsy** is often considered a definitive diagnostic step. It directly samples the endometrium for histological examination, which can diagnose endometrial cancer, hyperplasia, or atrophy. This procedure is critical for identifying the cause of postmenopausal bleeding.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While ultrasound can assess endometrial thickness and guide further management, it does not provide a tissue diagnosis.
- **Option B:** A Pap smear is a screening tool for cervical cancer and its precursors but does not directly investigate the cause of postmenopausal bleeding.
- **Option C:** Although hysteroscopy with dilation and curettage (D&C) can be diagnostic and therapeutic, it is generally considered after an initial assessment, not as the first line.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **postmenopausal bleeding is a red flag symptom** that requires prompt evaluation to rule out malignancy. An **endometrial thickness greater than 4-5 mm** on ultrasound in a postmenopausal woman is often considered abnormal and warrants further investigation, such as an endometrial biopsy.

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