Endocrine Pharmacology USMLE Step 1 Practice Question

A 54-year-old postmenopausal woman with estrogen receptor-positive breast cancer has been taking tamoxifen 20 mg daily for 2 years. She presents with vaginal bleeding and mild lower abdominal cramping (BP 128/82, HR 82, RR 16, temp 37°C). Pelvic ultrasound shows endometrial thickness of 12 mm, and endometrial biopsy reveals hyperplasia without atypia. Pelvic examination demonstrates a small amount of blood at the cervical os with no adnexal masses. She denies hormone replacement therapy use. Which selective estrogen receptor modulator is most likely responsible for this adverse effect?