We would probably say what K. Oranai first suggested:
You've got an elastic mark around your ankle. Are the socks too tight? Does it hurt?
or
The elastic in the socks has made a mark around your ankle. Are they too tight? Does it hurt?
Same meaning - you are just changing the subject in the first sentence.
In the second sentence though, note "The elastic in the socks..."
Words like "in" and other prepositions are some of the shortest words in English, but the most difficult to use. However, they are very important in spoken English. I'm going to try and put some more information about this up this weekend for everyone.
I'm just wondering whether an elastic mark can be a kind of mark which is able to stretch out or shrink back. This problem is still in my head. Would you mind explaining this to me and may be for the others as well. Thank you very much in advance.
Good question. An elastic mark refers to an imprint (mark) left on skin by a piece of clothing that contains elastic which was too tight. The mark itself doesn't stretch. "elastic" in this case is being used to describe how the mark happened.
I can see why you are confused. If we were to put "elastic" with "band" and get "an elastic band", in this case, "elastic" is used to describe the properties of the band.
In both cases, "elastic" is being used as an adjective, but for different purposes i.e. how something happened (mark) and something's physical properties (the band's properties).
If you still don't get it, tell me and I'll try and write a clearer answer.