67-year-old woman, sona has a 3 month history of increasing right shoulder pain. She is now unable to lift her arm to brush her hair or to take vessels off a shelf in the kitchen. She denies any numbness or tingling or radiation of the pain down her arm. On physical examination, she exhibits weakness in abduction and external rotation on her right arm. Her shoulder shows a normal passive range of motion. When asked to hold her arms abducted to 90 degrees, she is unable to do so on the right. There is no motor weakness in her forearm or hand. Her right upper extremity has normal pulse and normal sensation. Which of the following is the most likely diagnosis?