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1. The parents of a 6-month-old child who was normal at birth bring her into the clinic. Since their emigration to the U.S. from Eastern Europe soon after her birth, the child has developed diminished responsiveness, progressive blindness and deafness, and recently, seizures. Serum levels of which of the following compounds would be expected to be decreased in both of the parents?
A.
B.
C.
D.
E.