A 68-year-old man sustained a CVA and received a course of speech-language treatment for anomic aphasia. He was discharged after making rapid improvement early in therapy. Three years later his wife reports that he is having more difficulty speaking and understanding, but that his memory skills and orientation abilities remain intact. She has also noticed that his conversation skills are slowly deteriorating. Of the following, which is the most likely explanation for the client's communicative decline?
(A) A transient ischemic attack
(B) An astrocytoma, probably in the vicinity of the supramarginal gyrus
(C) Primary progressive aphasia
(D) Lewy body dementia