A 73-year-old male with a history of well-controlled hypertension presents with his wife, who reports that over the past 3 months he has become increasingly convinced she has been replaced by an identical-looking impostor. He acknowledges she looks exactly like his wife but insists 'something is off — she is not the real person.' He has also developed visual hallucinations of small animals, fluctuating attention, and his cognitive performance varies dramatically from hour to hour. Motor examination reveals mild bilateral resting tremor and cogwheel rigidity. His wife reports he has been physically acting out dreams during sleep for the past several years.