A 17-year-old previously healthy girl presents with complaints of progressive bilateral lower extremity weakness of 2 to 3 weeks’ duration. She is a competitive dancer who practices many hours each day. Upon initiation of intensive competition training, she notices increasing fatigue and loss of endurance, citing inability to kick her legs as high as usual. She describes generalized soreness in the quadriceps and hamstring muscles bilaterally, associated with weakness, occurring multiple times each day for the past 2 to 3 weeks. Acetaminophen and ibuprofen have had little effect.

She takes no medications and has a family history of rheumatoid arthritis and Crohn disease in her maternal grandmother. She denies alcohol or other substance use. She denies fever, headaches, blurry vision, respiratory symptoms, abdominal pain, diarrhea, vomiting, dysuria, hematuria, rashes, visual changes, changes in speech or swallowing, facial weakness or weakness performing activities of daily living. Physical examination is unremarkable, with...

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