Which metabolic disorder is a classic inborn error of metabolism that can cause cerebral dysfunction and requires dietary management?

• A. Cystic fibrosis
• B. Hemophilia
• C. Sickle cell disease
• D. Phenylketonuria

Answer: (D)

The idea here is recognizing a classic inborn error of metabolism where brain function can be protected by diet. Phenylketonuria arises from a deficiency of phenylalanine hydroxylase (or a related cofactor issue), causing phenylalanine to accumulate in the blood and brain. That buildup is neurotoxic, especially as the brain develops in infancy, leading to intellectual disability and other neurodevelopmental problems if untreated. The treatment is a lifelong diet that restricts phenylalanine, using specialized formulas and careful monitoring of levels to keep them in a safe range. Newborn screening detects the condition early, so dietary management begins before damage occurs, which is why this disorder is a classic example of an inborn error of metabolism managed by diet. The other listed disorders are not primarily metabolic blocks that cause cerebral dysfunction from toxic substrate buildup and are treated through different mechanisms (cystic fibrosis affects glands and lungs, hemophilia is a coagulation problem, and sickle cell disease is a blood disorder).