Psychiatrymiddle twist
A 13-year-old boy attends with his mother because his school has asked for a diagnosis. He has always been behind his peers and reads at a Year 3 level. He avoids eye contact, flaps his hands when excited and repeats phrases from television. He has a long narrow face with prominent ears, a high-arched palate and hyperextensible finger joints, and his testes are large for his age. Two of his mother's brothers attend a supported workplace. Which one of the following investigations is most likely to establish the underlying cause?