Physical examination of an infant delivered to a 42 year old, gravida 3, para 2, woman, is remarkable for slight hypotonia and a poor Moro reflex. Fuher examination reveals upslanting palpebral fissures, epicanthal folds, excess nuchal skin, an enlarged tongue, clinodactyly of the fifth fingers, and a single transverse palmar crease. Which of the following is the most likely diagnosis?