Review Question - QID 219502

A 47-year-old female with hypothyroidism presents with two and a half months of left shoulder pain that started insidiously. She has a considerable amount of pain at night that intermittently prevents her from sleeping, and over the last month has started to have difficulties at her job at the grocery store because she is unable to reach and stock shelves above shoulder level. On exam, she has a painful arc of motion that is limited to 80 degrees in the forward plane, and on passive examination she has a notable lack of external rotation that is painful and limited to 20 degrees compared to the contralateral side. Radiographs and an MRI obtained by her primary care physician are shown in Figures A and B, respectively. Which of the following cell lines is primarily responsible for the pathology accounting for the patient's physical examination and imaging findings?