A 32-year-old woman whose pregnancy was uncomplicated gives birth at term. A newborn physical examination shows a small lower lumbar skin dimple with a protruding tuft of hair. A radiograph shows that the underlying L4 vertebra has lack of closure of the posterior arches. What is the most likely diagnosis?
Arnold-Chiari malformation Dandy-Walker malformation Meningomyelocele Spina bifida occulta