Pyelonephritis: kidney infection

I saw a cat that came in with a fever and a painful abdomen. While collecting her urine and blood for some lab tests, I noted her urine was quite opaque. It's not rocket science: I thought she'd have pus in her urine. Her labwork came back with classic abnormalities for pyelonephritis: elevated white blood cell count (with the type of cell typically indicative of bacterial infection), mildly elevated urea, with pus and bacteria in the urine. In an earlier post, I discuss a case of pyelonephritis and prosatitis in a dog.
Where does the infection come from? The infection can start off in the bladder and ascend up to the kidneys, or it can arrive at the kidneys via the bloodstream from various sources: a bite, severe periodontal disease, etc, etc.
Treatment involves up to 6-8 weeks of antibiotics and in this case, hospitalization and iv fluids for at least a few days.
This kitty is responding well.

Gross.