A 10 year old boy is brought in by his parents because of behavioral problems leading to poor grades at school. He says he has uncontrollable persistent thoughts about how his possessions at school are out of order or messy. He constantly counts and checks them and refuses to interact with anyone else while he does so. He also hoards candies, action figurines, and erasers. Attempts to convince him not to do these activities don't work; he says there are "rules he must follow." His parents both have anxiety. One sister has ADHD. A brother has autism.
Challenge: What's the most likely diagnosis?