Polymyositis

A 52-year-old woman comes to the office with upper extremity weakness and joint pain. She reports that for two months, she has had difficulty reaching over her head to retrieve things from high shelves and brushing her hair. She has also been feeling fatigued and lost some weight lately. She takes only acetaminophen for muscle and joint aches. She reports no use of alcohol, tobacco, or illicit drugs. Her vital signs include temperature of 39ºC (102.2ºF), heart rate is 70/min, respirations are 18/min, and blood pressure is 135/87 mmHg. Muscle strength is decreased in the proximal upper extremities. Sensation and reflexes are intact globally. Skin examination is unremarkable. Creatine kinase,erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) are elevated. Which of the following is the most likely diagnosis?