Maybe you should read more carefully it says right in the article that he has Marfan's. I copy and pasted it below so you wouldn't miss it this time.

"Michael Phelps has a rare genetic disorder that has undoubtedly helped in his athletic endeavours, but that may also one day prove to be a curse. That disorder, which affects 1 in 5,000 people worldwide, is called Marfan's Syndrome"

One of the major definitive tests for the disease is an EKO, performed every year to monitor any growth from the disease. This disease is genetic, I have more than one family member who has been DIAGNOSED with it. So you might want to check your facts before you post something.