USMLE Answer 39

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39. The correct answer is E. The child has neonatal respiratory distress syndrome (hyaline membrane disease). This condition is caused by the inability of the immature lungs to synthesize adequate amounts of surfactant. Surfactant, which reduces surface tension, helps keep alveoli dry, and aids in expansion of the lungs, is synthesized by type II pneumocytes.

Alveolar capillary endothelial cells (choice A) are important in maintaining the capillary structure and permitting flow of gases into and out of the bloodstream.

Bronchial mucous cells (choice B) produce the usually thin (in healthy individuals) coat of mucus that lines the bronchi.

The ciliated bronchial respiratory epithelium (choice C) is responsible for moving the dust-coated mucus layer out of the bronchi.

Type I pneumocytes (choice D) are the squamous cells that line alveoli and permit easy gas exchange. These cells tend to be immature (and thick) in premature infants but are not the producers of surfactant.