Question grade : 10
12/12
A 6-year-old boy was brought to the hospital because of a two-month history of lassitude and weakness. At first, he had difficulty climbing stairs. For the last two months, he had to roll over on his side to get off the bed and use his hands on thighs to stand from the sitting position. For the last two weeks he asked for help to comb his hair. There are no other complaints and the family history was unremarkable. Routine vaccination was up to date. On examination, the child looked well, not pale, jaundiced or cyanosed and he was afebrile. His upper eye lids had purplish colored macules and the dorsal surface of his interphalangeal joints had flat-topped pinkish papules. His cheeks, forehead and the dorsal surfaces of the forearms were erythematous. Positive Gower sign was confirmed. Slight tenderness was also noted over the hamstring and quadriceps muscles. Tendon reflexes were normal and sensations were intact. Electromyography showed increased insertional activity, fibrillation potentials, sharp waves and decreased duration and amplitude of action potential. Motor and sensory conduction velocities were however, normal. Creatine kinase was 3000 U/L. Given this patient's presentation, what would be the most likely diagnosis?