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I am sorry you are worried about Lightening.
The dark urine can indicate dehydration, jaundice, infection, or internal bleeding.
Weight loss, lethargy and poor appetite are not specific for any one illness. Can you take his temperature? If you look at his gums, are the pink, or pale, or yellow tinged?
If I could, the place I would start is a thorough physical examination, check his temperature, etc. If his gums look pale at all, I would test him for feline leukemia virus.
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