Achilles tendon rupture

A 40-year-old man comes to the office for evaluation of right heel pain. He was playing tennis with his son when he experienced a sharp, stabbing pain of his right heel with an associated “popping” sound. The patient has been unable to ambulate since the injury occurred. Past medical history is significant for type 2 diabetes mellitus which is adequately managed with metformin. Vitals are within normal limits. Physical examination demonstrates a palpable gap two centimeters proximal to the right heel. The right leg does not yield plantar flexion with squeezing of the calf. Which of the following is the most likely cause of this patient’s condition?