A 5-day-old normally developed boy presents to the emergency department with vomiting and constipation. The mother states that the child has not passed stool since birth but only recently began vomiting. The vomitus has a greenish coloration. On examination, the abdomen is markedly distended and is dull to percussion. Digital rectal examination showsan empty rectum, but stool is passed explosively following the examination. A sweat chloride test is negative. The disorder in this infant arises from cells derived from which of the following embryologic cell populations?

Neural crest

15. The correct answer is C. The disorder described in the question is Hirschsprungs disease, a disorder of neural crest cells. Specifically, neural crest cells fail to migrate to distal portions of the colon, leading to a congenital lack of parasympathetic ganglion cells. This produces a functional obstruction of the colon, as peristalsis cannot take place. As a result, the abdomen becomes distended and bilious vomiting eventually results, while the rectum is empty on digital examination. Pathologic exam will reveal a lack of ganglia in Meissners and Auerbachs plexuses and nerve fiber hypertrophy in Meissners plexus. Answer A is incorrect. The endoderm gives rise to the epithelial lining of much of the gastrointestinal tract, but this is not a disorder of the epithelium. Answer B is incorrect. The mesoderm gives rise to smooth muscle, but this patient has Hirschsprungs disease, a disorder of colonic ganglion cells, not smooth muscle. Answer D is incorrect. The neuroectoderm gives rise to most components of the nervous system but not to the ganglia. Answer E is incorrect. The surface ectoderm gives rise to the epithelial lining of the lower anal canal, but this patient has Hirschsprungs disease, which is not a disorder of the epithelium.