This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
A critical factor in the regulation of craniofacial development 
Select and explain. 
A. Sonic hedgehog (SHH), B. Fibroblast Growth factor (FGFz), C. Wingless Drosophila melanogaster segment — Polarity gene (Wnt), D. Bone Morphogenic Protein (BMP)
A: Sonic hedgehog (SHH)
Answer: A: Sonic hedgehog (SHH) 
Explanation: None