A neonate is diagnosed with DiGeorge syndrome (22q11.2 deletion) presenting with hypocalcaemia, T-cell immunodeficiency, and conotruncal heart defects. What is the common embryological defect linking all these features?

• A Defect in the first pharyngeal arch mesoderm
• B Failure of Rathke's pouch invagination
• C Defective notochord induction affecting midline structures
• D Failure of neural crest cell migration into the 3rd and 4th pharyngeal pouches and conotruncus ✓

Explanation

DiGeorge syndrome results from defective migration of neural crest cells (specifically from the rhombencephalon) that populate the 3rd and 4th pharyngeal pouches and the conotruncal region. The 3rd pouch forms the inferior parathyroids and thymus; the 4th pouch forms the superior parathyroids and part of the thyroid C-cells. Failure of neural crest migration leads to absent/hypoplastic thymus (T-cell deficiency), absent/hypoplastic parathyroids (hypocalcaemia), and conotruncal heart defects (truncus arteriosus, tetralogy of Fallot, interrupted aortic arch). The neural crest also contributes to facial structures, explaining associated dysmorphic features.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.