A 12-year-old baseball pitcher describes progressive worsening of medial elbow pain on his throwing side. Examination reveals normal elbow range of motion. He is tender over the medial elbow to palpation. A dynamic ultrasound of his elbow shows no evidence of medial widening with valgus stress. His radiograph is shown in Figure A and an MRI is shown in Figure B. What is the most likely cause of his symptoms?