A previously healthy 13-year-old boy was referred because of fatigue and progressive weakness in the legs for one month. His right leg seemed more affected than the left one. He also had difficulty getting up from his bed and needed support of the arms. For one week, strength in the arms also decreased. When looking in the mirror, he had the impression that there was thinning of the upper arms and legs. He had lower back pain as well as nonradiating pain in the knee cavities, especially at night. He also had significant weight loss, from 40 to 35 kg. There had been no previous illness, skin changes, or insect bites.