Neoplasia USMLE Step 1 Practice Question

A 44-year-old woman presents to the emergency department with vaginal bleeding and pelvic pressure for 3 months. She denies fever, chills, or vaginal discharge. Past medical history is significant for multiple uterine fibroids diagnosed 5 years ago. Vital signs: BP 118/76, HR 88, RR 16, Temperature 37.2°C. Laboratory values reveal hemoglobin 9.2 g/dL (previously 12.5 g/dL one year ago). Pelvic ultrasound shows a 12 cm heterogeneous uterine mass with irregular borders, central areas of necrosis, and cystic degeneration, distinctly different in appearance from her known benign fibroids. Hysteroscopy with tissue biopsy is performed. Pathology demonstrates atypical spindle cells with marked nuclear pleomorphism, hyperchromatic nuclei, and >15 mitotic figures per 10 high-power fields. Immunohistochemistry is positive for smooth muscle actin and desmin. Which of the following is the most likely diagnosis?