A couple with no phenotypic symptoms of phenylketonuria have given birth to three children affected by the condition. What is the probability that they will have a phenotypically normal child in their future offspring?
Correct answer: C. 75%
Phenylketonuria (PKU) is an autosomal recessive disorder. If both parents are carriers, the probability of having a child affected by PKU is 25%, and the probability of having a phenotypically normal child is 75%. The couple has already had three affected children, which suggests they are likely carriers.