a 73 year old white female is brought in by her family with complaints of poor attention and memory problems that have been getting worse over the past several months. she denies recent headache, head trauma, fever, malaise, nausea and vomiting. she has a remote history of subarachnoid hemorrhage that did not require surgical intervention. she is otherwise very healthy for her age. there is no family history of dementia. neurologic exam reveals decreased attention and poor recent memory. her gait is wide based. what is the most likely diagnosis?