My daughter age6 has a very large unexplained bruise starting just under her knee and finishing above her ankle. She has Marian's syndrome.

Bruise. An unexplained bruise this large needs to be examined. Please get her to a health care worker soon.
Response. Hi Joe, Do you mean marfans syndrome? I have searched online and it looks like there may be a cross over in terms of Marian's to Marfans. If this is the case, Marfans syndrome, is a problem with connective tissue. Tissue that holds joints, stablizes blood supplies. The bruise around the knee joint maybe developing into a hematoma which is a small pocket of blood leaking into the weak tissue.