Review Question - QID 214401

A 48-year-old man is brought to his physician by his wife for "abnormal movements." Initially, his wife thought he was "fidgety" and "clumsy" as he would drop items such as a cup of water. Now his movements are described as "dance-like" where he tries to hide them by sitting on his hands or pretending that the movements were purposeful. His wife has noticed that he has become more irritable lately and that he has been less interactive with her, his grandchildren, and friends. He has a medical history of hypertension and hyperlipidemia adequately managed with lisinopril and simvastatin. His father is currently living and otherwise healthy. His mother committed suicide at age 51 for unclear reasons. Neurological examination is remarkable for chorea-like movements affecting his bilateral upper extremities. He appears apathetic and has impaired organization and planning on cognitive testing. An MRI of his brain is demonstrated in Figure A. Which of the following is the most likely cause of this patient's symptoms?