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18. The correct answer is C. The infant has Hirschsprung disease, which is due to an absence of ganglion cells in the wall of the colon. Neural crest cells contribute to the formation of many adult structures. Among these are all of the postganglionic neurons of the autonomic nervous system and the sensory neurons of the peripheral nervous system.

Ectoderm (choice A) forms the epidermis of the skin and the parenchymal cells of glands associated with the skin, such as the sweat glands, sebaceous glands, and mammary glands.

Endoderm (choice B) forms the epithelial lining of the gut tube and the parenchymal cells of glands associated with the gut tube, such as the liver and pancreas.

Neural ectoderm (choice D) forms the CNS, the somatic motor neurons of the peripheral nervous system, and the preganglionic neurons of the autonomic nervous system.

Splanchnic mesoderm (choice E) forms the visceral peritoneum, the visceral pleura, the visceral pericardium, and the stroma and muscle of the wall of the gut, among other structures.

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