**Core Concept**
Alzheimer's disease is a neurodegenerative disorder characterized by progressive neuronal loss and cognitive decline. The cerebral cortex, particularly the temporal and parietal lobes, is predominantly affected, leading to characteristic pathological changes.

**Why the Correct Answer is Right**
Neuronal loss in the cerebral cortex, particularly in the hippocampus and temporal lobe, is the hallmark of Alzheimer's disease. This is due to the accumulation of amyloid-beta plaques and neurofibrillary tangles, leading to neuronal dysfunction and eventual death. The cerebral cortex is responsible for higher-order cognitive functions, including memory, language, and executive function, which are typically impaired in patients with Alzheimer's disease. This neuronal loss is a result of the complex interplay between genetic and environmental factors, including the presence of apolipoprotein E (APOE) gene variants and age.

**Why Each Wrong Option is Incorrect**
**Option A:** Pigmentary degeneration in hippocampus is more characteristic of Parkinson's disease, which is primarily associated with motor symptoms and the presence of Lewy bodies.

**Option B:** Demyelination in cerebral cortex is a feature of multiple sclerosis, a chronic autoimmune disease that affects the central nervous system, leading to widespread demyelination and axonal loss.

**Option C:** Posterior column degeneration is associated with subacute combined degeneration, a condition caused by vitamin B12 deficiency, which affects the posterior columns of the spinal cord and leads to sensory ataxia.

**Clinical Pearl / High-Yield Fact**
Alzheimer's disease is the most common cause of dementia worldwide, accounting for approximately 60-80% of dementia cases. Early recognition and diagnosis are crucial for managing the symptoms and improving the quality of life for patients and their caregivers.

**✓ Correct Answer: D. Neuronal loss in cerebral cortex**