A 30 year old man is becoming concerned about the safety of his family. He has been checking the locks of the door every hour during the night. He becomes very anxious if his wife tries to stop him. What is the most likely diagnosis?

The key is D. OCD. [Obsessive compulsive disorder (OCD) is a mental health condition where a person has obsessive thoughts and compulsive activity. An obsession is an unwanted and unpleasant thought, image or urge that repeatedly enters a person's mind, causing feelings of anxiety, disgust or unease. A compulsion is a repetitive behaviour or mental act that someone feels they need to carry out to try to temporarily relieve the unpleasant feelings brought on by the obsessive thought].