Technically, mesenteric cysts are congenital in nature. However, if the 2 year clause is applicable in your case, you should be able to make the claim. Alternately, you may ask the doctor not to use the specific term of mesenteric cyst and instead use a non-specific term like abdominal cyst or intestinal cyst. Unfortunately, the insurance agencies are becoming very fussy nowadays and are rejecting claims at the slightest doubt.