As a young medical resident at Baylor College of Medicine in the Texas Medical Center in Houston in the 1980s, Richard Smith, M.D., saw a number of toddlers with large, disfiguring cysts on their face or necks. The cysts, called lymphatic malformations, or LMs, are gossamer thin veils of tissue filled up like water balloons. They’re usually benign but can have a profound impact on everything from...