Review Question - QID 212325

A 9-year-old boy presents to his orthopedic surgeon for ongoing evaluation of his lower extremity deformities. Specifically, his hips have been dislocating over the last year despite bracing in hip-knee-ankle-foot orthoses. He is paralyzed from below the L2 nerve root and has difficulty with both bladder and bowel incontinence. He has had these abnormalities since birth due to a lower back lesion that required surgical closure during infancy. Which of the following substances was most likely elevated in his mother's serum during her pregnancy?