**Core Concept**
Delusion of nihilism is a type of delusion where the individual believes that the world is ending, or that they themselves are insignificant or non-existent. Early morning insomnia is a common symptom in various psychiatric disorders, including depression and schizophrenia. This question requires the test-taker to recognize the characteristic symptoms of a specific psychiatric disorder.

**Why the Correct Answer is Right**
The correct answer is associated with Schizophrenia Spectrum Disorders, particularly with the diagnosis of Schizoaffective Disorder. Delusion of nihilism is a symptom of severe mental illness where the individual has a distorted view of reality. Early morning insomnia is a common feature in this disorder, often accompanied by other symptoms like hallucinations, disorganized thinking, and negative symptoms. The pathophysiology of schizophrenia is thought to involve abnormalities in dopamine and glutamate neurotransmission, as well as genetic predisposition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be tempting, but it is not the correct answer. While depression can cause early morning insomnia, it is not typically associated with delusion of nihilism.

**Option B:** This option is incorrect because delusion of nihilism is not a characteristic feature of this disorder. Although it can cause hallucinations and disorganized thinking, it is not typically associated with delusion of nihilism.

**Option D:** This option is incorrect because delusion of nihilism is not a characteristic feature of this disorder. While it can cause early morning insomnia, it is not typically associated with delusion of nihilism.

**Clinical Pearl / High-Yield Fact**
Delusion of nihilism is a classic symptom of severe mental illness, often seen in schizophrenia spectrum disorders. It is essential to recognize this symptom in the clinical context to provide appropriate treatment and support.

**Correct Answer: C. Schizoaffective Disorder**