**Core Concept**
Phenylketonuria (PKU) is a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH), which is essential for the metabolism of the amino acid phenylalanine (Phe). The condition leads to the accumulation of phenylalanine in the body, causing intellectual disability, seizures, and other neurological problems if left untreated.

**Why the Correct Answer is Right**
The correct answer is related to the inheritance pattern of PKU. PKU is an autosomal recessive disorder, meaning that a child must inherit two defective copies of the PAH gene (one from each parent) to develop the condition. Carriers of the disorder, who have one normal and one defective copy of the gene, are generally asymptomatic but can pass the defective gene to their offspring. If both parents are carriers, there is a 25% chance that each child will be affected with PKU, a 50% chance that each child will be a carrier, and a 25% chance that each child will be unaffected and not a carrier.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely related to the fact that children born to mothers with PKU may have microcephaly, but this is not the correct answer as it is a possible complication, not the exception.
**Option B:** This option is likely related to the fact that children born to mothers with PKU may have intellectual disability, but this is not the correct answer as it is a possible complication, not the exception.
**Option C:** This option is likely related to the fact that children born to mothers with PKU may have seizures, but this is not the correct answer as it is a possible complication, not the exception.

**Clinical Pearl / High-Yield Fact**
It is essential to note that early diagnosis and treatment of PKU can prevent intellectual disability and other complications. Babies born to mothers with PKU should be screened for the condition at birth, and if diagnosed, should be placed on a strict diet that limits phenylalanine intake.

**Correct Answer: D**