**Core Concept:** Delusions are fixed, false beliefs that persist despite contradictory evidence, and are an integral part of psychotic disorders such as schizophrenia. They are considered a cardinal symptom along with hallucinations, thought disorders, and motor abnormalities.

**Why the Correct Answer is Right:** Delusions are typically seen in disorders like schizophrenia, schizoaffective disorder, and delusional disorder. They are not typically found in disorders characterized by prominent affective symptoms, cognitive deficits, or motor abnormalities.

Delusions are not seen in disorders characterized by prominent affective symptoms like major depressive disorder or bipolar disorder. They are also not seen in disorders with cognitive deficits like dementia or frontotemporal lobar degeneration. Lastly, delusions are not found in disorders with motor abnormalities like Parkinson's disease or essential tremor.

**Why Each Wrong Option is Incorrect:**

A. Schizophrenia: Although delusions are a common feature in schizophrenia, they can coexist with prominent affective symptoms, making option A incorrect.

B. Dementia: Delusions are not a feature of dementia, as dementia is characterized by cognitive deficits and frontal-subcortical dementia, not delusions.

C. Major Depressive Disorder: Delusions are not a feature of major depressive disorder, as the primary symptom in major depressive disorder is mood disturbance, not delusions.

D. Parkinson's Disease: Delusions are not a feature of Parkinson's disease, which is characterized by motor abnormalities like tremors, rigidity, and bradykinesia.

**Clinical Pearl:** Understanding the core features of different psychiatric disorders helps in distinguishing them from one another and in formulating an accurate diagnosis. This knowledge is essential for appropriate treatment and management of patients presenting with psychotic symptoms.