A newborn develops tetanic spasms with serum calcium of 6.5 mg/dL. Echocardiography reveals truncus arteriosus. Chest imaging shows absence of thymic shadow, and flow cytometry shows profoundly reduced CD3+ T cells with normal B cell numbers. What is the most likely genetic finding?

• A Deletion at 22q11.2 affecting development of the third and fourth pharyngeal pouches ✓
• B Mutation in BTK gene on the X chromosome
• C Mutation in the WAS gene causing defective actin cytoskeleton regulation
• D Deletion of the ITGB2 gene encoding the beta-2 integrin chain

Explanation

The triad of neonatal hypocalcemic tetany, conotruncal cardiac defect, and T-cell lymphopenia defines DiGeorge syndrome, caused by microdeletion of 22q11.2 leading to failed development of third and fourth pharyngeal pouch derivatives: thymus and parathyroids. Option B causes X-linked agammaglobulinemia, which features absent B cells and normal parathyroid function, inconsistent with the hypocalcemia and cardiac lesion seen here. Recognizing the associated cardiac anomaly plus hypocalcemia clinches the diagnosis over isolated T-cell defects.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.