A 52-year-old woman with a history of smoking and uncontrolled hypertension presents to the emergency department with the worst headache of her life, onset during Valsalva while straining at stool. Non-contrast CT head shows hyperdense blood in the basal cisterns in a characteristic star-pattern, marked as **A** in the diagram. Which of the following aneurysm locations is MOST likely responsible for this radiologic finding?