Review Question - QID 219828

A 17-year-old high school pitcher presents with shoulder pain that occurs consistently during the phase of throwing denoted in Figure A. An MRI of the affected shoulder demonstrates the changes shown in Figures B and C. On clinical exam, the patient complains primarily of posterior shoulder pain and has a difference of ~30 degrees of internal rotation of the shoulder when stabilized in 90 degrees of abduction, though his total arc of motion between both shoulders is preserved. Which of the following structures denoted in Figure D is most likely pathologically thickened in this setting?