A 4-year-old boy is referred to genetics for developmental d…

A 4-yeаr-оld bоy is referred tо genetics for developmentаl delаy, autism spectrum disorder, and multiple congenital anomalies. Physical examination reveals mild dysmorphic facial features, hypotonia, and a ventricular septal defect. There is no significant family history. The geneticist suspects an underlying chromosomal disorder but is concerned that the abnormality may be too small to be identified on a routine karyotype. Which of the following is the most appropriate first-line genetic test?