A 44-year-old man comes to the physician after noticing his right pupil looks larger than his left. He thinks it has been this way for several weeks. He has no pain and no restriction of his eye movements When examined in a darkly lit room, both pupils are 4mm in size. When the lights are turned on. his right pupil dilates to 4mm and his left pupil constricts to 2mm in size. His eye movements are normal as is the rest of his neurological examination. He does not have a drooping eyelid. This physical finding is due to which of the following pathological processes?