1. From the Heart Hospital (A.S.F., M.P.H., P.M.E., C.M., J.C.M.), University College London Hospitals National Health Service Trust; Department of Medicine (A.S.F., P.M.E., C.M., J.C.M.), University College London; Department of Histopathology (M.T.A.), Great Ormond Street Hospital for Children National Health Service Trust; Cardiovascular Unit (M.S.H., A.M.T.), University College London Institute of Child Health; and Great Ormond Street Hospital for Children (M.S.H., A.M.T.), London, United Kingdom.