Alzheimer's is the most common cause of dementia, a general term for memory loss and other cognitive abilities serious enough to interfere with daily life. Alzheimer's disease accounts for 60 percent to 80 percent of dementia cases.
In the case of Nacy Pelosi, the disease is manifesting in her inability to distinguish between Bush and Trump.
Under Bush, in 2006, she opposed the Secure Fence Act... that is manifesting today in her opposition to the next stage in funding the Fence -- something Trump has referred to as a wall.
Pelosi is still arguing against Bush -- she has lost about twelve years. She does recover, but the gaff is consistent with a level of mental degradation which raises serious concerns over her competence to be in Congress, much less be Speaker of the House.