Which of the following is a birth defect characterized by a hole in the spinal cord?

Myelomeningocele is a type of spina bifida, which is a birth defect that occurs when the spinal column does not close completely during fetal development. In myelomeningocele, the defect is characterized by a protrusion of the spinal cord and its surrounding membranes through the opening in the vertebrae, resulting in a visible sac on the back. This condition can lead to significant neurological effects, including paralysis and issues with bowel and bladder control, due to the exposure of the spinal cord to the external environment.

While meningocele also involves a defect in the spinal column, it specifically refers to a protrusion of the protective membranes (the meninges) without including the spinal cord itself, making it less severe compared to myelomeningocele. Hemangioma is a benign tumor made up of blood vessels and is unrelated to spinal defects. Polydactylism is a genetic condition characterized by extra fingers or toes and does not pertain to spinal cord abnormalities.

Thus, myelomeningocele is correctly identified as the birth defect characterized by a hole in the spinal cord, making it the most relevant answer in this context.