Suggest Medication For Swelling In The Right Knee

hi doctor... i would like to know the answer for the case below...a ten year old boy came to the orthopeadic with chief complaint swollen in the right knee due to trauma one week ago. the swollen has already developed before trauma and pain has already developed several months before. patients body weight felt decrease one month ago. no fever.
1) what is the planning diagnosis that you sugest?2) what is the differential diagnosis ?thank you