You are at thanksgiving dinner and your Aunt Muriel (yes, we all have an Aunt Muriel) tells you that she is having a great deal of pain in her shoulder that is in the
anterolateral portion of the shoulder and radiates to the elbow. You palpate her shoulder and find pinpoint pain just inferior to the acromium process. This all
happened after she recently got back into tennis 3x/week. What is the most likely structure injured based on this history and presentation?