A 3 years old boy is brought to his pediatrician because his parents think he does not seem to be“ developing normally” The boy＇s mother states that her son seemed normal for at least the first two years of his life. He was walking and beginning to speak in sentences. He was able to play with his brothers. The mother has been noticing that over the past three months her son has markedly lost these previously acquired abilities. He will no longer play with anyone else and has stopped speaking entirely. He has almost lost all bowel control, when previously he had not needed a diaper for at least a year. Which of the following is the most likely diagnosis