Skin rash and muscle weakness

A 48-year-old Hispanic woman came to the clinic as a new patient--her chief complaint was a rash that appeared on her face 3 months before and had recently spread to her chest and hands. It itched occasionally and seemed to worsen after exposure to the sun. She also said that for the last month she had been feeling very weak-she had difficulty rising from a seated position and walking up the stairs to her apartment. She also felt as if her arms were heavy, making it difficult for her to brush and dry her hair in the morning. The patient was otherwise healthy with no known medical conditions, and she was not taking any medications. Her family history was noncontributory. What is your diagnosis? What diagnostic tests would you order for confirmation?