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What is your son's age?Has he had this particular problem before?Is there any history of trauma, and any other reliant medical history except gout?
Any pain/ fever/ rash?
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In this situation I would suggest that you can wait for a day, and if the problem worsens tomorrow, then you need to be checked by the GP for possible tests.
I cannot be sure as to the cause of the problem, but it may be related to a mild trauma like twisting of the knee, that can cause some inflammation,
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