Bursitis

A 39-year-old man presents to the clinic with worsening swelling and pain in his left knee. The pain has been ongoing for weeks and worsens with knee flexion. The patient works as a plumber and spends most of the day on his knees working in crawl spaces. He has a history of hypertension and diabetes and takes amlodipine and metformin daily. The patient smokes half a pack of cigarettes per day and drinks three beers on the weekends. Temperature is 37.0°C (98.6°F), pulse is 93/min, respirations are 12/min, blood pressure is 146/75 mmHg, and O₂ saturation is 95% on room air. The patient can fully flex and extend the knee and bear weight without significant pain. Physical examination is notable for the findings seen below, and a radiograph of the left knee is also shown. Which of the following is the most likely diagnosis?
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