In response to a complaint by her kindergarten teacher, a mother brings her 4-year-old girl to her pediatrician. The child has been noticed spending time alone while at kindergarten, talking to her self and to imaginary friends. The conversations also occur at home. Her reading and writing are comparable to her classmates. At her bedside, she keeps a pillow and blanket for her companion. Her mother also reports that her child has frequent nightmares, sometimes waking her in the night in tears. The mother wants her child evaluated further. Stem- What is the most likely cause?