This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Musculoskeletal  disorders, Subject name: Pediatrics
Pinku, a 9-year old girl, has difficulty in combing her hair and climbig up stairs since 6 months. On examination, Gower sign is positive and maculopapular rash present over MCP joints. What is the next appropriate investigation to be done ? 
Select and explain. 
A. RA factor, B. Serum creatine kinase, C. Electromyography, D. Muscle biopsy
B: Serum creatine kinase
Answer: B: Serum creatine kinase 
Explanation: Proximal muscle weakness and maculopapular rsah over the metacarpophalangeal joints (Gottron papules) are suggestive of dermatomyositis. As it is an inflammatory myopathy, the next investigation is serum creatine kinase. The gold standard investigation is an muscle biopsy. Ref: nelson textbook of pediatrics 21st edition PGno:1283