Which mental health disorder is primarily characterized by recurrent and intrusive thoughts?

The primary characteristic of Obsessive-Compulsive Disorder (OCD) is the presence of recurrent and intrusive thoughts, known as obsessions. Individuals with OCD experience persistent thoughts, images, or urges that are often distressing and anxiety-provoking. These obsessions compel them to perform certain behaviors or mental acts, known as compulsions, in an attempt to reduce the anxiety associated with the intrusive thoughts. This cycle of obsessions and compulsions is a hallmark feature of OCD.

In contrast, while Major Depressive Disorder can involve ruminative thinking, it's not specifically defined by intrusive thoughts. Generalized Anxiety Disorder focuses on excessive worry about various aspects of life without the focused obsessions seen in OCD, and Bipolar Disorder is characterized by mood swings that include periods of depression and mania, rather than the specific cognitive patterns seen in OCD. Thus, the defining feature of recurrent and intrusive thoughts makes Obsessive-Compulsive Disorder the correct choice in this context.