A 54-year-old man is brought to the office by his friend because of worsening memory impairment over the past several months. The patient is also withdrawn and not focused during conversations. He is slow to answer questions or carry out instructions. The patient has a prolonged history of HIV infection and has had several opportunistic infections due to inconsistent antiretroviral medication use. On physical examination, he is alert and oriented. Muscle strength, deep tendon reflexes, and sensation are normal. MRI of the head shows diffuse cerebral atrophy with no focal mass lesions or areas of demyelination.