A 66 year old man presents with abnormal behavior for 1 month. He has become highly suspicious of his neighbors, without just cause. Further questioning reveals a history of worsening forgetfulness for 6 months, including losing his way in his own neighborhood 3 times.

His medical, psychiatric, and family histories are unremarkable, and he is not on any drugs or over-the-counter medications. He does not smoke, drink, or abuse recreational drugs.

He consumes a normal diet, including red meat and fish. He has been sexually promiscuous throughout his life, with the most recent relationship just 2 years ago.

He underwent a routine check-up 3 months earlier, at which time a complete blood count, serum electrolyte assay, and renal, thyroid, and liver profiles were all found to be normal.