Thyroid Disorders USMLE Step 1 Practice Question

A 78-year-old woman presents with a 2-month history of rapidly enlarging neck mass, progressive dysphagia, and hoarseness. Vital signs show BP 138/82, HR 92, RR 18, temp 37.2°C, SpO2 98%. She denies recent weight loss. CT imaging demonstrates an invasive thyroid tumor with local compression of the trachea and esophagus. TSH is elevated at 8.2 mIU/L. Biopsy reveals highly pleomorphic undifferentiated giant cells. Which diagnosis is most likely?