A 10-year-old boy complains of muscle pain and difficulty climbing stairs and getting dressed. According to the patient's history, the complaints first arose 4 months ago, but lately he has developed intensified muscle pain, low appetite, and difficulty swallowing. Objectively, facial edema, purple periorbital erythema, and desquamation of the skin of the abdomen and torso are observed. What is the most likely diagnosis in this case?