Patellar tendon rupture

A 61-year-old Caucasian man comes to the office because of acute onset of knee pain and swelling. His medical history is relevant for Sjogren's disease and a recent community-acquired pneumonia. He currently takes prednisone and levofloxacin. Upon further interrogation, the patient mentions that he was walking down the street when he suddenly felt a "snapping" sound and severe pain on his left knee. Physical exam shows localized tenderness and palpable pain on his left knee, in addition to, a "floating" displaced patella and limited knee extension. His temperature is 36.7°C (97.9°F), pulse is 87/min, respirations are 18/min, blood pressure is 130/60 mmHg. A conventional knee radiograph is obtained and shown below.    



Which of the following is the most likely diagnosis?