**Question:** Delusion is not seen in

A. Schizophrenia
B. Bipolar disorder
C. Major depressive disorder
D. Acute organic brain syndrome

**Core Concept:** Delusions are false beliefs that are not based on reality and are resistant to logical argument or evidence. They are a core feature of certain psychiatric disorders.

**Why the Correct Answer is Right:** Delusions are often seen in schizophrenia, where they are one of the cardinal symptoms along with hallucinations, disorganized thinking, and motor behavior. In contrast, delusions are not the main feature in bipolar disorder, where mood swings, hypomania, or mania are the predominant symptoms. Major depressive disorder focuses on mood symptoms, including sadness, hopelessness, and anhedonia. Acute organic brain syndrome refers to a sudden impairment of mental function due to an underlying organic cause, such as stroke or encephalitis, which may or may not include delusions.

**Why Each Wrong Option is Incorrect:**

A. Schizophrenia: Delusions are present in schizophrenia, making this a wrong answer choice.
B. Bipolar disorder: Although delusions can occur in bipolar disorder, they are not the primary feature, making this a wrong answer choice.
C. Major depressive disorder: Delusions are not a core feature of major depressive disorder, making this a wrong answer choice.
D. Acute organic brain syndrome: Delusions can be present in acute organic brain syndromes, but they are not the defining feature of the condition, making this a wrong answer choice.

**Clinical Pearl:** Delusions are an essential feature of schizophrenia, distinguishing it from other psychiatric disorders and emphasizing the importance of a comprehensive diagnostic approach when assessing patients with psychiatric symptoms.