Thanks to everybody for your advice and encouragement regarding Zsa Zsa's troubles. After many trips to the vet -- including one enforced "cat daycare" in order to get a fresh urine sample -- the diagnosis is...
...inconclusive. Instead of knowing what she's suffering from, we at least have a better idea of what she ISN'T suffering from.
She doesn't have worms, for instance, which is particularly weird considering what I pulled out of her butt last week. It must have been a piece of string...or maybe a bit of undigested mouse.
She also doesn't have diabetes, and nothing is wrong with her thyroid. That's the good news.
The bad news is that her urine shows signs of approaching kidney problems...or maybe liver problems, they're not sure which. Fortunately these problems can be managed through proper diet, so we're now doing the three-day "New Cat Food Taste Test," which is kind of fun and which she absolutely LOVES.
I am instructed to keep watching her for further deterioration, and to buy a bathroom scale so I can monitor her weight. I had previously refused to get a scale because I suspect it would lead to compulsive weighing of MYSELF, but there are certain sacrifices one must make for one's unfailing companion.
As for Zsa Zsa herself, she's back to her normal behaviour, though alarmingly thin. She's just as social, active, curious, and thirsty as she always was. Her "bad spell" during the weekend may have been due to a number of things -- like the terrible heat and the worm medication I gave her -- but she SEEMED to get bad whenever I fed her Purina soft food, which I kept doing in order to comfort her. She may have developed an intolerance for something in that food. Very, very strange.
Anyway, Zsa Zsa no longer appears to be on death's door and is instead scratching back at mine. Thank goodness!