Which initial diagnostic test is preferred for evaluating painless vaginal bleeding in a postmenopausal patient?

Transvaginal ultrasound is the preferred initial diagnostic test for evaluating painless vaginal bleeding in a postmenopausal patient due to its effectiveness in assessing the uterine lining and identifying potential causes of abnormal bleeding. In postmenopausal women, any episode of vaginal bleeding is concerning for conditions such as endometrial hyperplasia or endometrial cancer. Transvaginal ultrasound offers a non-invasive method to measure the thickness of the endometrial stripe, which is crucial in determining the need for further investigation or intervention.

Advanced imaging techniques, such as CT of the abdomen and pelvis, although they might provide more comprehensive views of the pelvic organs, are not the first line in this situation since they expose patients to radiation and are less specific for endometrial pathology. Colposcopy and hysteroscopy are more invasive procedures used for direct visual examination of the cervix and the inside of the uterus, respectively, but they are typically reserved for cases where ultrasound findings indicate significant abnormalities or when there is a need for biopsy. Thus, transvaginal ultrasound stands out as the most suitable and non-invasive initial diagnostic tool in this clinical scenario.