Medical Genetics Inability to remove excess phenylalanine from the blood during infancy and early childhood produces a variety of problems including mental retardation. Fortunately, a simple test (needing only a drop of blood ) done shortly after birth can identify the genetic defect and with close attention to the amount of phenylalanine in their diet the children can develop normallyMedical Genetics Inability to remove excess phenylalanine from the blood during infancy and early childhood produces a variety of problems including mental retardation. Fortunately, a simple test (needing only a drop of blood) done shortly after birth can identify the genetic defect and, with close attention to the amount of phenylalanine in their diet, the children can develop normally