At a routine check-up, a 7-year-old boy is found to have osteoporosis. The patient is tall and thin with pale skin, fair hair, and flushed cheeks. He has arachnodactyly, pes cavus, and bilaterally dislocated lenses, and demonstrates developmental delay with mild mental retardation. His mother is told that her child might benefit from folic acid supplementation. Which is the most appropriate test to confirm the diagnosis?
(A) Enzymatic assay for the enzyme HGPRT
(B) Genetic studies demonstrating a mutation in type I collagen
© Genetic studies indicating >200 copies of the CGG trinucleotide repeat on the X chromosome
(D) Nitroblue tetrazolium test
(E) Nitroprusside cyanide test