I think I know now what happened to Tucker.
I asked my veterinarians to review the notes, lab results, and x-rays taken by the emergency vet. One of them sat down with me last week, and we hashed out a theory that seems to explain all the medical signs and symptoms.
We think Tucker died from a perforated ulcer, which would have let the extremely toxic contents of his digestive tract out into his abdomen. ("Acute peritonitis" is a possible diagnosis.)
There's no direct proof of an ulcer, but there's a hint of leakage on an x-ray. And the sudden onset of acute symptoms, the extremely high fever, pain in the belly....
In a vet hospital modeled after a human hospital, maybe Tucker could have been saved. It might have required a radiologist to give a better reading of the x-rays, an internist to endoscopically examine and treat the perforation, and a surgeon to open the abdomen and clean out and treat the poisoning. In other words, a team working just on Tucker.
But this clinic had a single vet in charge of more than 30 overnight patients, emergency and not. Her goal was just to stabilize Tucker and keep him alive until the specialists came in in the morning, and that wasn't possible.
Whether she thought of perforated ulcer, I don't know. She didn't mention it, even when the high fever and belly pain led me to ask about something sharp causing a puncture and then peritonitis.
So why the perforation if there wasn't anything hard and sharp in there?
Steroids. Specifically, 3½ years of near-daily doses of prednisolone to help his breathing. Tucker's respiratory system was permanently crippled by the pneumonia and other lung problems that he had when I got him.
So that's the theory, and I'm grateful to my vets for coming up with it. It might be wrong, but I'm still grateful and very comforted that there is an explanation to hold onto.
Logically - when I'm able to think about it logically - I expect my senior pets to die before me. But if they suffer, dammit, I want to know why.