A 59-year-old man is brought to the office by his wife because of depressed mood and increased forgetfulness for the past 4 months. His wife notes that during this time, he has had difficulty remembering his daily schedule at work. He has smoked one pack of cigarettes daily for 30 years. Vital signs are within normal limits. His Mini-Mental State Examination score is 22/30. Examination discloses involuntary muscle twitching. On neurologic examination, deep tendon reflexes are 3+ bilaterally. Babinski sign is present bilaterally. Electroencephalography shows triphasic periodic sharp wave complexes. An MRI of the brain shows signal hyperintensity in the basal ganglia and temporal and frontal cortices. Histologic analysis of the patient's brain tissue would most likely show accumulation of which of the following?

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Eductional Objective: Creutzfeldt-Jakob disease is a prion disease characterized by rapidly progressive dementia, myoclonus, and death within a year of symptom onset.  It is caused by the accumulation of an abnormally folded (ie, beta pleated sheet) protein that is resistant to proteases and can trigger similar conformational changes in other normally folded proteins.

A 59-Year-Old Man Is Brought to the Office by His