Journal #5696

A mother brings in her 4-year-old boy because she is concerned about his increasing clumsiness. He has been previously healthy and achieved developmental milestones on time. His growth parameters are normal. On physical examination, his mental status is normal, as are results of cranial nerve and sensory examinations and reflexes. However, he cannot rise from the floor without using his hands, and his running looks clumsy.
Of the following, the MOST appropriate next test to assess the cause of this child's symptoms is

electromyography

lumbar puncture

measurement of serum creatine kinase

muscle biopsy

spine magnetic resonance imaging