**Core Concept**
Alzheimer's disease is a progressive neurodegenerative disorder characterized by the accumulation of amyloid plaques and neurofibrillary tangles, leading to neuronal loss and cognitive decline. The hallmark clinical feature is severe, progressive impairment of memory and thinking abilities.

**Why the Correct Answer is Right**
Dementia is the core diagnostic criterion for Alzheimer's disease, encompassing deficits in memory, reasoning, and daily functioning. While delirium and delusions can occur in other conditions, they are not defining features of Alzheimer's. Delirium refers to acute confusion, often due to infection or medication, and delusions are false beliefs seen in psychotic disorders. Alzheimer’s primarily presents with **progressive, irreversible dementia**, not acute confusion or fixed false beliefs.

**Why Each Wrong Option is Incorrect**
Option A: Delirium is an acute state of confusion with fluctuating levels of consciousness, typically due to medical illness or intoxication. It is not a feature of Alzheimer’s disease, which has a chronic, progressive course.
Option B: Delusions are persistent false beliefs, commonly seen in schizophrenia or delusional disorder, not in Alzheimer’s. While some patients may develop paranoid thoughts, this is not a primary or defining feature.
Option D: "All of the above" is incorrect because delirium and delusions are not core features of Alzheimer’s; only dementia is consistently and pathologically linked.

**Clinical Pearl / High-Yield Fact**
Remember: **Alzheimer’s = Dementia**, not delirium or delusions. Dementia is the only consistent, defining feature. Always differentiate between acute confusion (delirium) and chronic cognitive decline (dementia).

✓ Correct Answer: C. Dementia