Which type of disorder is characterized by repetitive, intrusive thoughts?

The type of disorder characterized by repetitive, intrusive thoughts is Obsessive-Compulsive Disorder (OCD). In OCD, individuals experience obsessions that are persistent and unwanted thoughts, images, or urges that cause significant anxiety or distress. These obsessive thoughts often lead to compulsive behaviors or mental acts aimed at reducing this anxiety. The hallmark of OCD is this cycle of obsession and compulsion, where the thoughts are intrusive and unwanted, significantly affecting the individual's daily functioning.

In contrast, while Post-Traumatic Stress Disorder (PTSD) involves intrusive memories related to a traumatic event, it is primarily concerned with re-experiencing the trauma rather than the repetitive, unwanted thoughts seen in OCD. Schizophrenia is a serious mental illness characterized by distorted thinking, perceptions, and emotional responsiveness, not specifically by repetitive intrusive thoughts. Generalized Anxiety Disorder (GAD) involves excessive worry about various aspects of life but is not defined by the specific, intrusive thoughts that characterize OCD. Therefore, OCD is the correct answer as it directly aligns with the description of the disorder being characterized by such thoughts.