<prompt> The commonest cause of limp in a child of seven years is -T.B. hipC.D.HPerthes diseaseSlipped upper femoral epiphysis  <response>: The most common cause of limp in a child between 3-10 years is transient synovitis of hip.However, among the given options Perthes disease is the best answer as Perthes disease is the second most common cause of limp in a child between 5-10 years. 