A 21-year-old recreational lacrosse player complains of four months of gradually worsening left shoulder pain. She states that her symptoms are primarily recreated when overhand shooting, and she localizes her pain to the posteromedial border of the scapula. On exam, she exhibits tenderness to palpation and a doughy fullness about the inferomedial border of the scapula with palpable crepitus. She has no appreciable weakness on external rotation testing with the elbow at the side, but with forward flexion of the affected extremity the inferior medial scapula protrudes posteriorly and medially. She undergoes 3 months of postural and scapular strengthening exercises followed by an injection with no significant improvement in her symptoms, and is now scheduled for surgery. What is the most likely diagnosis?