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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 reported 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?
A. Aromatic amino acid metabolism
B. Branched chain amino acid metabolism
C. Purine metabolism
D. Pyrimidine metabolism
E. Sulfur-containing amino acid metabolism
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c... (lesch nyhan's syndrome)?
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The correct answer is C. The disease is Lesch-Nyhan syndrome, and the yellow-orange crystals of uric acid in
the diaper are an important, but often neglected, clue to early diagnosis. Lesch-Nyhan syndrome is
characterized by a tremendous overproduction of purines, because the reutilization of purines via the purine
salvage pathway is blocked by a near total absence of hypoxanthine-guanine phosphoribosyl-transferase
(HGPRT) 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 arthritis or gouty nephropathy.
Phenylketonuria is an example of a disorder of aromatic amino acid metabolism (choice A) characterized by
mental retardation.
Maple syrup urine disease is an example of a disorder of branched chain amino acids (choice B) causing motor
abnormalities and seizures.
Orotic aciduria is an example of a disorder of pyrimidine metabolism (choice D), characterized by retarded
growth and development as well as megaloblastic anemia.
Homocystinuria is an example of a disorder of sulfur-containing amino acids (choice E), characterized by mental
retardation, dislocation of the lenses, osteoporosis, and thromboses.
