Alzheimer Hippocampal Atrophy

A 72-year-old retired schoolteacher presents with 2 years of progressive short-term memory loss, misplacing items, and word-finding difficulty. She became lost in a familiar neighborhood last month. MMSE is 18/30. Gait is preserved, and there are no visual hallucinations or behavioral changes. T1-weighted coronal MRI through the temporal lobes shows the pattern marked **A** in the diagram — bilateral hippocampal and medial temporal atrophy with widening of the choroidal fissure and enlargement of the temporal horns. Parietal and posterior cingulate cortices show mild atrophy; frontal and occipital lobes are preserved. Which of the following pathological hallmarks is most directly responsible for the neuronal loss and atrophy pattern seen at **A**?