Review Question - QID 214522

A 54-year-old man presents to his primary care provider due to a 3-month history of left thigh pain. This pain began without any known injury and has slowly gotten worse in the past several weeks. It has also been associated with some thigh swelling. He takes ibuprofen and acetaminophen to help with the pain. On exam, the patient’s temperature is 98.4°F (36.9°C), blood pressure is 130/82 mmHg, pulse is 72/min, and respirations are 12/min. He weighs 160 pounds, which is down from 178 pounds at his last visit. On his left medial thigh is a well-circumscribed palpable mass, approximately 6 cm in diameter. There is no exquisite tenderness to palpation. After imaging workup, the patient undergoes excision of the mass, with histopathology shown in Figure A. Which of the following is the likely diagnosis?