Q. A couple brings their 4-month-old son for his pediatric wellness exam. The child was born with a birth defect that was diagnosed in utero. The prenatal history reveals that the mother had an uncomplicated pregnancy but did not take prenatal supplements, as she was unaware of her pregnancy for the first few months. On physical examination, a small dimple with a tuft of hair is noted over the lower spine. His lower extremities appear weaker compared to his upper limbs. The diagnosis of spina bifida was established with further workup. Given the child's presentation and prenatal history, which of the following factors most likely contributed to the child's condition?