A 10 year old male, although mentally retarded, is able to carry out activities of daily living, including feeding and dressing himself. On physical examination, he has branchycephaly and oblique palpebral fissures with prominent epicanthal folds. On the palm of each hand is seen a transverse crease. On auscultation of the chest, there is a grade III/IV systolic murmur. Which of the following diseases will be most likely have by the age of 20 -