Allan Gallup, a retired lawyer and businessman, grew increasingly forgetful in his last few years. Eventually, he could no longer remember how to use a computer or the television. Although he needed a catheter, he kept forgetting and pulling it out.

It was Alzheimer’s disease, the doctors said. So after Mr. Gallup died in 2017 at age 87, his brain was sent to Washington University in St. Louis to be examined as part of a national study of the disease.

But it wasn’t just Alzheimer’s disease, the researchers found. Although Mr. Gallup’s brain had all the hallmarks — plaques made of one abnormal protein and tangled strings of another — the tissue also contained clumps of proteins called Lewy bodies, as well as signs of silent strokes. Each of these, too, is a cause of dementia.

Mr. Gallup’s brain was typical for an elderly patient with dementia. Although almost all of these patients are given a diagnosis of Alzheimer’s disease, nearly every one of them has a mixture of brain abnormalities.