A 3-year-old boy is brought to the pediatric clinic with a 2-year history of rapid weight gain, insatiable appetite, and behavioral rigidity. On examination, he has almond-shaped eyes, a narrow bifrontal diameter, small hands and feet, and significant central obesity. His parents report he was hypotonic and had poor feeding as a newborn. Genetic testing confirms the abnormality marked **A** in the diagram. Which of the following is the most appropriate first-tier diagnostic test to confirm the underlying genomic imprinting disorder?