A male infant presents with dextrocardia, a left-sided liver and a right-sided stomach. Chest radiograph shows situs inversus totalis, and nasal biopsy reveals absent dynein arms in respiratory cilia. The primary defect responsible for the abnormal visceral situs lies in:

• A Failure of neural crest migration into the outflow tract
• B Ciliary rotation at the primitive node during gastrulation ✓
• C Defective HOXD13 expression in the lateral plate mesoderm
• D Loss of FGF8 secretion from the apical ectodermal ridge

Explanation

Left-right asymmetry is initiated by clockwise rotational beating of monocilia at the primitive node, which drives leftward flow of morphogens and triggers NODAL, LEFTY and PITX2 expression on the left side. Primary ciliary dyskinesia with dynein arm defects randomises this flow, producing situs inversus or heterotaxy, classically with Kartagener syndrome. Neural crest, HOXD13 and AER-FGF8 pathways do not set visceral laterality.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.