DiGeorge syndrome results from failure of development of the 3rd and 4th pharyngeal pouches. The primary immunological defect is:

• A Thymic aplasia causing T cell deficiency with relatively normal B cells ✓
• B Absent B cells with failure of antibody production (X-linked agammaglobulinemia pattern)
• C Combined T and B cell deficiency due to ADA enzyme deficiency
• D Defective phagocyte oxidative burst causing failure to kill catalase-positive organisms

Explanation

DiGeorge syndrome (22q11.2 deletion) is characterized by thymic aplasia or hypoplasia, causing profound T-cell deficiency — T cells cannot mature without the thymic environment. B-cell numbers are relatively normal but humoral immunity is secondarily impaired because T-cell help for B cells is absent. Classic features are the CATCH-22 mnemonic: Cardiac defects, Abnormal facies, Thymic aplasia, Cleft palate, Hypocalcemia. ADA deficiency causes SCID with combined T and B cell deficiency; CGD involves phagocyte oxidative burst failure.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.