A newborn girl is found to have absent thymic shadow on chest X-ray, hypocalcemia (due to absence of parathyroid glands), and conotruncal cardiac defects (truncus arteriosus). T-cell count is severely reduced, but B-cells are normal. Serum immunoglobulins are low (due to absent T-helper function). Which embryological structure failed to develop properly?

• A Bone marrow stromal cells
• B Lymph sac endothelium
• C Second pharyngeal arch
• D Third and fourth pharyngeal pouches ✓

Explanation

DiGeorge syndrome (22q11.2 deletion) results from failure of neural crest cell migration into the 3rd and 4th pharyngeal pouches (branchial arches), which normally give rise to the thymus (T-cell education) and inferior parathyroid glands (calcium regulation). Absence of thymus = T-cell immunodeficiency (susceptibility to viral/fungal infections, opportunistic organisms). Absence of parathyroid = hypoparathyroidism/hypocalcemia. Conotruncal defects arise from failure of neural crest contribution to the cardiac outflow tract. B-cells are normal but are non-functional without T-cell help.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.