Cat losing weight, vet says one kidney is large. What's next?

There are a number of causes of weight loss in cats. I'm assuming that the information of 1 kidney being larger than the other was something that the vet noted when the belly was palpated on exam. That finding, in and of itself, doesn't necessarily mean the kidneys are the issue. I see that Neko is only 3 years old. Weight loss in a younger cat could be due to kidney failure, though its much more common in senior cats.
Working only off of the information that Neko is a young cat who losing weight and has 1 kidney larger than the other, with no other history given, it is hard for us to get any impression for what may be wrong. Again, if she were a senior cat I would suspect kidney failure, and I would then recommend a canned prescription kidney diet, lots of water, TLC, and know that time is limited -- with the understanding that we don't KNOW its the kidneys without blood work. However, since Neko is young, its harder to jump to this sort of advice. It could be her kidneys, but other concerns would include inflammatory bowel disease, intestinal worms, or even something like FIP (feline infectious peritonitis). Some causes are relatively inexpensive to treat, and others are much more difficult. Additional information about any other signs would help us-- is she vomiting, how is her appetite, is she playful and active or is she dumpy and lethargic, is she hiding, etc.
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