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I am sorry to hear you are worried about Simba. My name is Rebecca. I am a veterinarian and will do my best to help.
Does he go outside at all? One of the most common reasons for a cat to have a swollen leg, is from an infection from being bitten by another cat.
The moderator is correct, a broken bone is a possibility. Will he put any weight on the leg? If it is a broken leg, he probably would not walk on it at all.
Infection or a fracture are the most likely reason. Another possibility is a blood clot in the leg preventing normal circulation but we usually do not see a lot of swelling with that.
Is he eating? Can you take his temperature with a thermometer? If this is an infection he might have a fever, 103 or higher.
Let me know what other questions you have.