**Core Concept**
Delusions are fixed, false beliefs that are not amenable to change in light of conflicting evidence. They are a hallmark symptom of several psychiatric disorders and can be distinguished from overvalued ideas, which are beliefs that are excessive or irrational but not necessarily false.

**Why the Correct Answer is Right**
Delusions often arise from a combination of factors, including cognitive distortions, environmental influences, and underlying psychiatric conditions. They can be persecutory, grandiose, or referential in nature. The most common psychiatric disorders associated with delusions are schizophrenia, bipolar disorder, and major depressive disorder with psychotic features. In these conditions, delusions can be caused by abnormalities in neurotransmitter systems, particularly dopamine and serotonin.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because delusions are not typically seen in anxiety disorders, such as generalized anxiety disorder or panic disorder. While anxiety disorders can involve excessive worry or fear, they do not typically involve fixed, false beliefs.

**Option B:**
This option is incorrect because delusions are not characteristic of dissociative disorders, such as dissociative identity disorder. While dissociative disorders can involve disruptions in consciousness or memory, they do not typically involve fixed, false beliefs.

**Option C:**
This option is incorrect because delusions are not typically seen in personality disorders, such as borderline personality disorder or antisocial personality disorder. While personality disorders can involve distortions in thinking or behavior, they do not typically involve fixed, false beliefs.

**Clinical Pearl / High-Yield Fact**
To distinguish between delusions and overvalued ideas, consider the following: delusions are fixed, false beliefs that are not amenable to change in light of conflicting evidence, whereas overvalued ideas are excessive or irrational beliefs that may be open to revision.

**Correct Answer: D. Schizophrenia**