When Ludivine Verboogen and Romain Alderweireldt’s third child was born in Belgium in late 2015, they marveled at his long fingers. Perhaps one day he will be a famous pianist, they thought. But soon Ludivine grew worried that her son was not developing as well as his two older sisters had. His muscles seemed weak, and the physiotherapy appointments she was taking him to three times a week didn’t seem to be helping. “A lot of doctors were telling us that he was fine, nothing was wrong with him,”