A newborn has cyanotic heart disease. Echocardiography shows truncus arteriosus. Chest imaging shows absence of the thymic shadow, serum calcium is 6.8 mg/dL, and lymphocyte count is very low. Which investigation will confirm the diagnosis?

• A DNA methylation analysis of chromosome 15q11-q13
• B Karyotype for trisomy 21
• C Measurement of 7-dehydrocholesterol levels
• D Fluorescence in situ hybridization for 22q11.2 deletion ✓

Explanation

The combination of a conotruncal defect, thymic aplasia causing T-cell deficiency, and hypocalcemia from parathyroid hypoplasia defines DiGeorge syndrome, part of the 22q11.2 deletion spectrum. The deletion is submicroscopic and invisible on routine karyotyping, so confirmation requires FISH or chromosomal microarray targeting 22q11.2. Trisomy 21 screening is irrelevant here because truncus arteriosus with immunodeficiency points squarely to the del22q11 phenotype.

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.