A 47-year-old woman presents to her primary care physician, concerned that she has had a stroke or has some sort of neurologic illness. She is experiencing difficulty "finding her words," is being what she describes as "scatterbrained," and is bumping into things as well as having trouble going up the stairs in her home. She first noticed her symptoms 6 months ago. They have gotten progressively worse during that time. Her history is otherwise unremarkable. She has no other underlying comorbidities and no history of head trauma or known neurologic disease, and she does not take any medications. She is married and has two teenage sons. She was the primary caregiver to her father, who had Alzheimer's disease (AD) and died 5 years ago at age 70 years. Her older brother was diagnosed with AD 2 years ago at 50 years of age.

Physical exam reveals mild anomic aphasia during conversation, difficulty with spatial awareness, and trouble remaining focused on the questions she is being asked. There is nothing remarkable about her physical appearance. Heart, breath, and bowel sounds are all normal on auscultation. Blood pressure is 120/80 mm Hg. The following laboratory testing is performed: complete blood cell count, cobalamin, liver function tests, thyroid-stimulating hormone, and rapid plasma reagin. All tests show nothing remarkable, with values within normal range. HIV serology is negative.