“The reason the skull grows is to make room for the growing brain,” the neuropathologist explained. “Everything is packed really tightly. Nature doesn’t leave any gaps.”

The septum pellucidum, a small membrane between the two halves of the brain, was atrophied to the point that it looked withered and fragile, even perforated. When the neuropathologist later went to look for others in a similar condition, the youngest comparable example was a 46-year-old boxer.

The fornix, a C-shaped bundle of nerves, was similarly deteriorated, stripped of its relative heft. The hippocampus, too. Even some of the most famously diseased brains that the neuropathologist had explored, from men who had died decades later, did not have such obvious signs of destruction when examined by the naked eye.

But only under a microscope could the disease be diagnosed with certainty. Wafer-like tissues were immunostained, using antibodies designed to discolor a specific protein — in this case, tau, which clumps and spreads, killing brain cells. That is where the full scope of the damage was apparent.

Tau, stained brown, appeared like bursts of fireworks in the frontal cortex, the part of the brain that controls decision making, impulse and inhibition. The neuropathologist could see it spreading through the brain. It was in the amygdala, the part of the brain that regulates emotions like fear and anxiety, and the temporal lobe. She spotted “a perfect demonstration” of lesions around the tiny blood vessels, a telltale sign. She found previous microhemorrhages and astrocytic scarring around the ventricles.

She declared the case Stage 3 on her own scale of severity, which goes from 1 to 4. It was the most damage she had seen in anyone that age. Among the hundreds of other brains she had examined and graded, the median age of a Stage 3 brain from his profession was 67. Now she had one that was only 27.