A neonate has tetany within the first week of life. Evaluation shows serum calcium of 6.8 mg/dL, an absent thymic shadow on chest radiograph, lymphocyte count of 1200/mm3 with low CD4 count, and echocardiography showing truncus arteriosus. Which genetic test will most likely confirm the diagnosis?

• A Karyotype showing 45,X/46,XY mosaicism
• B Mutation analysis of the TBX5 gene
• C DNA methylation analysis of chromosome 15q11-q13
• D FISH for deletion at 22q11.2 ✓

Explanation

The combination of hypocalcemic tetany, thymic aplasia with T-cell deficiency, and conotruncal cardiac defect is DiGeorge syndrome, caused by a microdeletion at 22q11.2 best detected by FISH. Methylation analysis of 15q11-q13 diagnoses Prader-Willi or Angelman syndrome, TBX5 mutations cause Holt-Oram syndrome, and 45,X mosaicism relates to Turner syndrome, none of which produce this triad.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.