**Core Concept**
Obsessive-compulsive neurosis, now referred to as obsessive-compulsive disorder (OCD), is a psychiatric condition characterized by recurring, intrusive thoughts (obsessions) and repetitive behaviors (compulsions) that an individual feels compelled to perform. The underlying pathophysiology involves abnormalities in brain regions responsible for mood regulation, such as the orbitofrontal cortex and the anterior cingulate gyrus.

**Why the Correct Answer is Right**
The patient with OCD is at an increased risk of developing other anxiety disorders, such as major depressive disorder (MDD), generalized anxiety disorder (GAD), and panic disorder. This is due to the shared underlying neurobiological mechanisms, including hyperactivity in the amygdala and altered serotonin levels. Specifically, the serotonin system, which plays a crucial role in mood regulation, is known to be dysregulated in individuals with OCD. This dysregulation can contribute to the development of comorbid mood disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the typical comorbidities seen in OCD patients.
**Option B:** While substance abuse can be a comorbidity in OCD patients, it is not the most likely association.
**Option C:** This option is incorrect as OCD is not typically associated with psychosis.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that patients with OCD often exhibit comorbidities with other psychiatric conditions, and a comprehensive treatment plan should address these associated conditions.

**Correct Answer: D. Major Depressive Disorder (MDD)**