Maybe OP does not want to go through the hassles of verifying his ID because asides that, I've never heard of an exchange asking where any coin came from.
I have used a number of exchanges and never came across any that asks for source of coin before honoring my orders, so it is either OP is having something to hide or the association of the coins with Mtgox raised a red flag. I fault the op choice of words, they sound like half truth to me.
I wonder how I am supposed to sell bitcoins on an exchange. I have been considering several and have gone through the KYC/AML process for three exchanges. The problem is once I sell my coins they might ask me to prove where I got them from. Keep in mind I am not intending to withdraw the FIAT, I just want to sell high and rebuy lower, and withdraw the coins.
The problem with proving the source of my coins is, how the hell do I do it? I bought them on MtGox and they can't ask MtGox for verification. Even if they did, the coins have gone through several addresses over the years. I can't even trace it back to the MtGox purchase since they went through other exchanges (and got mixed).
So what is one supposed to do here?
I suppose you deposited though a wallet, it history should be a pointer to the source if that is really how it got to Mtgox in the first place.