Embryology USMLE Step 1 Practice Question
A 2-day-old male neonate presents with a small notch in the transitional zone and incomplete vermillion border of the upper lip. Vital signs are stable: BP 65/40 mmHg, HR 145 bpm, RR 38/min, temperature 37.2°C, SpO2 98% on room air. Physical examination reveals no cleft palate or other facial anomalies. Prenatal ultrasound showed normal amniotic fluid volume. Which embryologic failure during weeks 6-8 of gestation most likely explains this lip malformation?
Answer choices
- AAbnormal development of the third and fourth pharyngeal pouches
- BInsufficient migration of neural crest cells
- CFailure of the maxillary processes and medial nasal swellings to fuseCorrect answer
- DIncomplete joining of the palatine shelves
- EBony defects of the malar bone and mandible
- FFailure of the lateral nasal processes to merge with the maxillary processes
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