A 62 year old woman complains of difficulty remembering important dates and appointments. She also describes poor concentration, daytime sleepiness and easy fatiguability. She is concerned about her forgetfulness because her mother suffered from recurrent strokes and had severe memory loss. Her father died of chronic leukemia. Her daughter's recent job loss has caused her a lot of stress. She does not smoke or consume alcohol. Her appetite is decreased but she has gained 4 pounds over the last three months. She visited an otolaryngologist for hoarseness of recent onset. She takes over-the-counter laxatives for constipation and occasional aspirin for knee pain. She denies any other medication use. Which of the following is the most likely diagnosis in this patient?