AKT-5264

A 65-year-old woman has had a fall. She has been well all her life and has rarely had to see the doctor. She smokes 10 cigarettes a week. Examination is unremarkable other than a left-sided ptosis and some slight thinning of the muscles of her left hand, which she thinks might be long-standing.
What is the most probable reason for her left-sided ptosis?