This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
An 8-month-old child is brought to a pediatrician because of the mother's concern about the boy's tendency to compulsively bite his fingers. On questioning, the mother repoed that she has noticed yellow-orange crystals in his diapers, but has not mentioned them to anyone. A genetic defect in which of the following pathways should be suspected? 
Select and explain. 
A. Aromatic amino acid metabolism, B. Branched chain amino acid metabolism, C. Purine metabolism, D. Pyrimidine metabolism
C: Purine metabolism
Answer: C: Purine metabolism 
Explanation: The condition is Lesch-Nyhan syndrome, and the yellow-orange crystals of uric acid in the diaper are an impoant, but often neglected, clue to early diagnosis. Lesch-Nyhan syndrome is characterized by a tremendous overproduction of purines, because the reutilization of purines the purine salvage pathway is blocked by a near total absence of hypoxanthine-guanine phosphoribosyl-transferase (HGP) activity. Patients with this severe X-linked disease, for reasons that are unknown, show aggressive behavior that leads to self-mutilation. They may also develop gouty ahritis or gouty nephropathy. Phenylketonuria is an example of a disorder of aromatic amino acid metabolism characterized by mental retardation. Maple syrup urine disease is an example of a disorder of branched chain amino acids causing motor abnormalities and seizures. Orotic aciduria is an example of a disorder of pyrimidine metabolism, characterized by retarded growth and development as well as megaloblastic anemia.