Embryology USMLE Step 1 Practice Question
A 5-year-old boy is brought to clinic by his parents for evaluation of developmental concerns. He was born with severe micrognathia and glossoptosis, which necessitated emergency tracheostomy on the first day of life to relieve upper airway obstruction. Physical examination reveals a small, receding mandible, a U-shaped cleft palate, and bilateral knee and elbow contractures. Imaging confirms mandibular hypoplasia. The combination of these findings is consistent with Robin sequence. Which of the following best describes the primary embryologic mechanism underlying this condition?
Answer choices
- AFailure of neural crest cell migration into the first pharyngeal arch during weeks 4-6 of gestation
- BInsufficient growth and elongation of the mandibular process during the first trimesterCorrect answer
- CAbnormal fusion of the palatal shelves with the vomer and nasal septum
- DFailure of Meckel cartilage to serve as a template for mandibular bone formation
- EPremature differentiation of maxillary process tissue, preventing mandibular expansion
- FDefective neural crest-derived mesenchyme in the second pharyngeal arch
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