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Baseball is back! You are seeing a 13 year old boy who plays year-round baseball with a tournament team as a pitcher. He has been having more and more right shoulder pain with throwing over the last few months. He, his family, and his coach are concerned because his pitch velocity and accuracy have worsened. He cannot localize the pain well, but is tender on palpation of the proximal lateral humerus. He has full range of motion and no neurovascular deficits. Radiographs of bilateral shoulders show the right proximal humeral physis widened compared to the left. 

What is the likely diagnosis?