A previously healthy 50 year old woman comes to the physician because of double vision for three days. Her temperature is 37 C (98.6 F). The patient denies nausea or vomiting. Examination reveals ptosis and slight divergence of the right eye. Extraocular movements are limited in all directions, except laterally. The right pupil is larger than the left and poorly reactive to light. Examination of the fundus fails to reveal papilledema. Which of the following is the most likely underlying condition?