A 9-year-old boy is referred to a pediatric clinic by his school psychologist because of poor academic performance, cognitive delay, and hyperkinetic behavior. Family history is significant for early dementia and ataxia in the maternal father. Physical examination reveals that the boy has a long thin face with prominent ears, some facial asymmetry, and a prominent forehead and jaw. His vital signs are normal, his lungs are clear to auscultation, and heart sounds are normal. His abdomen is soft, nontender, and nondistended. Examination of the extremities showed hyperextensible finger joints. The examining physician suspects a possible genetic disorder. What would be the best test to diagnose this disease? A. Brain MRI B. Cytogenetic testing for fragile X C. Developmental evaluation by a speech/language therapist D. EEG E. Measurement of testicular volume F. Southern blot analysis