If you haven't already refrigerated the Lavender chick you found dead this morn, do so asap. (Don't freeze her body, only refrigerate.) Since you now have a body to send in for an official necropsy at your state lab, there is no more need to have a live marek's test. The bonus is that a very thorough necropsy will be performed looking for all abnormalities and possible reasons for death, including but not limited to Marek's. That includes various worm species. You will fill out a standard form to submit along with the carcass, but don't hesitate to include a separate page, listing all details you think may be relevant. (Same as with fellow members here, the more info you give to vets at your state lab, the better.) If I was in your shoes and believed Marek's was the only disease within my flock, I might not be as quick to send a body in. But since you acquired this chick (as well as others) as an older started pullet, there's truly no telling what avian diseases she was exposed to before she was sold to you. Though Mareks Disease is untreatable, some other diseases are, which is why it's important to know as much as possible in order to best manage the health of your flock.