A 68-year-old woman is evaluated in the outpatient clinic for cognitive concerns. Her husband reports that over the past 18 months, she has gradually become forgetful, misplaces objects frequently, and has difficulty managing finances. However, she remains alert and oriented, converses normally, and her attention span is intact. She performs activities of daily living independently. Neuropsychological testing confirms memory impairment with preserved attention and executive function. MRI brain shows mild cortical atrophy. On examination today, she is calm, cooperative, and her vital signs are normal. What is the most likely diagnosis?