A young man evaluated for infertility has chronic sinusitis, bronchiectasis, and complete situs inversus with dextrocardia. The embryological basis of his abnormal organ laterality is dysfunction of:

• A Gap junctional coupling between visceral endoderm cells
• B Motile monocilia at the primitive node generating directional flow ✓
• C The apical ectodermal ridge of the lateral body folds
• D Neural crest cells migrating through pharyngeal arch 3

Explanation

Left-right asymmetry is initiated by clockwise rotating motile cilia at the primitive node, which create leftward flow of extraembryonic fluid and trigger the Nodal-LEFTY-PITX2 cascade on the left side. In Kartagener syndrome, dynein arm defects make these nodal cilia immotile, so sidedness is assigned randomly, giving situs inversus in roughly half of cases alongside the respiratory and sperm tail defects. The other options describe structures unrelated to laterality specification.

Reference: Langman's Medical Embryology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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