A 56-year-old woman comes to the physician for a 4-month history of gradually progressive weakness in her shoulders and hips. Vitals are within normal limits. BMI is 20. Physical examination shows mild bilateral weakness in the deltoids, glutei, and quadriceps muscles. ESR and creatine kinase are elevated. Her brother has rheumatoid arthritis. She traveled to Mexico one month ago. Which of the following is the most appropriate next step in diagnosis?
Electromyography and nerve conduction studies
Liver biopsy
MRI of the cervical spine
Muscle biopsy
Stool examination
