A neonate with Down syndrome is found to have hepatosplenomegaly and a violaceous skin infiltrate. Her FBC shows WBC 27.2 × 109/l, Hb 139 g/l and platelet count 27 × 109/l. A blood ûlm shows blast cells, some of which have basophilic cytoplasm and cytoplasmic blebs, and are thought likely to be megakaryoblasts. There are occasional micromega-karyocytes, nucleated red cells and granulocyte precursors. It is concluded that the skin lesion is leukaemia cutis. The most appropriate management is: