**Core Concept**
Eugen Bleuler, a Swiss psychiatrist, introduced the concept of the '4 A's of schizophrenia to describe the primary symptoms of the disorder. These 'A's are: **Ambivalence**, **Affect flattening**, **Association disturbance**, and **Autism**.

**Why the Correct Answer is Right**
The 4 A's of Bleuler are fundamental in understanding the pathophysiology of schizophrenia. **Ambivalence** refers to the coexistence of contradictory emotions or ideas, **Affect flattening** is a reduction in emotional expression, **Association disturbance** is a disorganized thought process, and **Autism** is a withdrawal from social interactions. These symptoms are a hallmark of schizophrenia and are used to diagnose the disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** Ambivalence is indeed one of the '4 A's of Bleuler, so this option is incorrect.
**Option B:** Affect flattening is another of the '4 A's of Bleuler, so this option is incorrect.
**Option C:** Association disturbance is also one of the '4 A's of Bleuler, so this option is incorrect.
**Option D:** Delusions are a symptom of schizophrenia but are not one of the '4 A's of Bleuler.

**Clinical Pearl / High-Yield Fact**
When diagnosing schizophrenia, it's essential to note the presence of the '4 A's of Bleuler, as they are a crucial component of the diagnostic criteria. A mnemonic to remember the '4 A's is: "Ambivalence, Affect, Association, Autism".

**Correct Answer:** D. Delusions