Zola, the African orphan and would-be daughter of Meredith Grey (Ellen Pompeo) and Derek Shepherd (Patrick Dempsey), suffers from spina bifida, which is a neural tube disorder. The neural tube is an embryonic structure that eventually develops into the brain, spinal column, and their surrounding tissues. Normally, the neural tube finishes developing and closes by the 28th day after conception, but the process fails to complete in fetuses with spina bifida. This causes defects in the baby's spinal column and backbone, even open lesions.
Varying degrees of paralysis in the lower limbs is also a possibility with spina bifida; surgery is the only treatment for severe cases. In Zola's case, the spina bifida resulted in hydrocephalus — an excessive build-up of fluid in the brain. Recent studies have suggested that women who consume 0.4 mg of folic acid everyday are significantly less likely to bear children with neural tube disorders.
Sources: Mayo Clinic, National Institute of Neurological Disorders and Stroke