Hi Xue,
This one is tricky for me,
I am not sure what the answer is,
The medial joint line certainly has an anechoic area surrounded by an echogenic area.
The appearance is unfamiliar to me,
The growth plates look OK sonographically,
Plain radiographs and an MRI might be needed to be more specific,
I presume this appearance is unilateral and there is nothing similar on the other side.
It is a bit tricky for me to orientate on the images as well,
Is the MCL normal and this area seperate to it?
Was there a clinical swelling or skin changes?
The anechoic area seems adjacent to the meniscus, however i am not sure I have seen a para-meniscal cyst in a kid.
If you have any other images to share or x-ray,/ MRI information please share.
Steve