Q. A 76-year-old female presents to your clinic with complaints of left lower extremity pain. The patient reports the symptoms began 2 or 3 months ago. She denies any known history of trauma or previous problems with that knee. She states the pain occurs primarily with walking and improves with rest, and reports experiencing cramping in her left calf while walking, which causes her to frequently stop and rest. The patient states that the symptoms have been gradually worsening over time and are affecting her ability to perform daily activities. Her past medical history is significant for hypertension, hyperlipidemia, and gout. She is a lifelong smoker and does not consume alcohol. On physical examination, she is afebrile, her blood pressure is 145/90 mm Hg, pulse 72/min, respiratory rate 20/min, and oxygen saturation is 97 percent on room air. The patient appears to be in moderate distress and has a limp when she walks. Her left knee has a mild effusion and is tender to palpation over the medial joint line. There is no crepitus or deformity observed. No masses are palpated. The patient has a strong left femoral pulse, and her left ABI is found to be 0.7. There is mild weakness noted on ankle dorsiflexion and eversion on the left side. You order an x-ray of her left knee (see image). Based on the information you have at this point in time, which of the following is the most likely cause of this patient’s pain?