**Core Concept**
The investigation of choice for postmenopausal bleeding involves evaluating the endometrium for malignancy or precancerous lesions. Postmenopausal bleeding is an abnormal vaginal bleeding that occurs after 12 months of amenorrhea in women who have reached menopause.

**Why the Correct Answer is Right**
Endometrial sampling or biopsy is the gold standard for diagnosing endometrial cancer or hyperplasia in postmenopausal women. It involves taking a small sample of endometrial tissue for histopathological examination. This procedure helps to identify the cause of bleeding and guides further management. The American College of Obstetricians and Gynecologists (ACOG) recommends endometrial sampling in all postmenopausal women with abnormal vaginal bleeding.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pap smear is not the primary investigation for postmenopausal bleeding. While it can help identify cervical cancer or precancerous lesions, it is not specific for endometrial pathology.
* **Option B:** Ultrasound can help identify endometrial thickening or polyps but is not diagnostic. It may be used in conjunction with endometrial sampling but is not the investigation of choice.
* **Option C:** Hysteroscopy can visualize the uterine cavity and diagnose endometrial lesions but is more invasive than endometrial sampling and is usually reserved for cases where sampling is inconclusive or not possible.

**Clinical Pearl / High-Yield Fact**
Postmenopausal bleeding is a symptom that requires prompt evaluation to exclude endometrial cancer, which is a common malignancy in this age group. The risk of endometrial cancer increases with age, obesity, and unopposed estrogen exposure.

**Correct Answer:** C. Endometrial sampling or biopsy.