AKT-5101

You come across a 79-year-old woman who has a medical history of diabetes, osteoarthritis, and hypertension. She experienced pain while bearing weight after twisting her leg while getting out of a car. The pain has reduced with simple analgesia. She also mentions a lump under her knee. During the examination, you notice a non-tender 4 cm lump just below the popliteal fossa that becomes tense when the leg is extended. The patient has full power throughout. What could be the most probable diagnosis?