Final answer:
The accumulation of beta-amyloid plaques in the brain is the underlying cause of Alzheimer's disease.
Step-by-step explanation:
The underlying cause of Alzheimer's disease, a type of senile amyloidosis, is the accumulation of beta-amyloid plaques in the brain. These plaques are dense conglomerations of a protein called beta-amyloid that is not functioning correctly. The plaques disrupt normal brain function and lead to the death and degeneration of brain cells, resulting in the progressive memory loss and dementia characteristic of Alzheimer's disease.