Final answer:
Vascular neurocognitive disorder is characterized by small strokes affecting cognition, whereas Alzheimer's disease features senile plaques and neurofibrillary tangles leading to neuron degeneration, particularly in the hippocampus, causing cognitive decline.
Step-by-step explanation:
The defining characteristics of vascular neurocognitive disorder and Alzheimer's disease differ in their etiology and histopathological features. Vascular neurocognitive disorder is characterized primarily by the impact of small strokes or other vascular problems on cognitive function. This condition results from problems in the blood supply to the brain, which can lead to areas of dead or damaged brain tissue. In contrast, Alzheimer's disease is known for two hallmark pathological features: the presence of senile plaques and neurofibrillary tangles. Senile plaques are the abnormal clumps of beta-amyloid protein fragments that form between neurons, while neurofibrillary tangles are twisted strands of the tau protein found inside neurons. Together, these abnormalities contribute to the degeneration of brain cells, synapse loss, and significant brain atrophy, particularly affecting the hippocampus and leading to cognitive decline.