A 10-year-old boy displays deficits in intellectual functions that involve his ability to reason, solve problems, and learn from instruction or experience. He has been slow to develop skills in school and is behind his peers in reading, writing, and mathematical abilities. As a result, he has significant impairments in his ability to develop socially appropriate interpersonal communication skills and has developed no real friendships. He is dependent on others for assistance with his basic needs, such as dressing and feeding. Which of the following diagnoses most accurately represents this presentation?