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?

Spina bifida occulta is the mildest form of neural tube defect. It is characterized by defective closure of the vertebral arches, but with intact meninges and spinal cord, and "closed" by skin and connective tissue. A radiograph may show its presence in 20% of the population. Despite the lack of an open defect, there is an increased risk for meningitis. In an Arnold-Chiari malformation, there is a small posterior fossa and extension of the cerebellum into the foramen magnum and a lumbar meningomyelocele. Dandy-Walker malformation is detected by ultrasound evidence of a cyst in the fourth ventricle and agenesis of the cerebellar vermis. Meningomyeloceles are open neural tube defects. Tuberous sclerosis is a rare disease that causes firm hamartomatous "tubers" in the cortex, and is not associated with neural tube defects.