**Core Concept**
Nihilistic delusions are a type of psychotic symptom characterized by a false belief that oneself or others are dead, nonexistent, or somehow irrelevant to the world. This concept falls under the realm of abnormal psychology and psychiatry, specifically within the domain of thought disorders.

**Why the Correct Answer is Right**
Nihilistic delusions are typically associated with severe mental health conditions, such as schizophrenia or bipolar disorder. These delusions often arise from the disintegration of thought processes and can be linked to the disruption of neurotransmitter systems, particularly dopamine and serotonin. The exact pathophysiology of nihilistic delusions is complex and not fully understood, but they are thought to be related to abnormalities in the prefrontal cortex and other brain regions involved in executive function and reality testing.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because nihilistic delusions are not typically seen in anxiety disorders, such as generalized anxiety disorder or panic disorder. While anxiety disorders can feature delusions, these are usually centered around specific themes, such as fear of harm or persecution.
**Option B:** This option is incorrect because nihilistic delusions are not characteristic of obsessive-compulsive disorder (OCD). OCD is marked by intrusive thoughts and compulsions to perform specific rituals or behaviors, but it does not typically involve delusions of nonexistence or irrelevance.
**Option C:** This option is incorrect because nihilistic delusions are not a hallmark feature of post-traumatic stress disorder (PTSD). PTSD is characterized by flashbacks, nightmares, and avoidance behaviors related to a traumatic event, but it does not typically involve delusions of nonexistence or irrelevance.

**Clinical Pearl / High-Yield Fact**
Nihilistic delusions can be a clue to underlying severe mental illness, particularly schizophrenia or bipolar disorder. When encountering patients with nihilistic delusions, it's essential to conduct a thorough psychiatric evaluation to identify the underlying condition and develop an effective treatment plan.

**Correct Answer:** D. Schizophrenia