You are answering a USMLE-style medical question. Question: A 70-year-old woman with no significant medical history begins to experience memory loss and personality changes. Over the next few months, her symptoms become more severe, as she experiences rapid mental deterioration. She also starts to have sudden, jerking movements in response to being startled and gait disturbances. Eventually, she lapses into a coma and dies eight months after the onset of symptoms. What process likely caused this woman's illness? Options: A) Loss of dopaminergic neurons in the substantia nigra pars compacta. B) Autoimmune inflammation and demyelination of the peripheral nervous system. C) Conversion of a protein from an a-helix to a ß-pleated form, which resists degradation. D) Frontotemporal atrophy and the accumulation of intracellular, aggregated tau protein. Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.