45 year old woman presented with 12-month history of proximal muscle weakness in lower extremities, including difficulty climbing stairs and rising from seated position. She had frequent falls due to legs giving out. She noticed mild droopiness of the left eyelid since 3 years before, but had no diplopia or swallowing problems.  She also reported intermittent myalgia and muscle tenderness. PMH was positive for well controlled diabetes and hypothyroidism. Family history was significant for postoperative death of her mother after a surgery that involved general anesthesia, as she developed hyperthermia and rhabdomyolysis. Exam showed mild L.sided ptosis, 4/5 weakness of bilateral hip flexion and knee extension, and trace to absent knee jerks (other DTRs including ankle jerks were 2+). Previous labs: CK 1700-3000 during the prior 2 years, AST 68 (8-37), ALT 52 (8-35),  Myasthenia Gravis/Lambert Eaton panel: (-),  Lactic acid: 1.6 (Normal). Click here for nerve conduction study and EMG as well as MRI of the thigh muscles. What is the next diagnostic step? diagnosis?