A 3-Year-Old Boy Is Brought to the Office Due to Abnormal

A 3-year-old boy is brought to the office due to abnormal motor development.  He was born at 40 weeks gestation and had an unremarkable perinatal course.  The boy developed normally during the first year of life.  However, for the past 2 years, he has had progressive bilateral leg stiffness and abnormal involuntary movements.  His cognitive and motor development is also delayed.  There is no significant family history of neurological or muscular disorders.  The patient's height, weight, and head circumference are below the 3rd percentile.  Examination shows bilateral spastic paresis of his lower extremities and frequent choreoathetoid movements.  Comprehensive laboratory testing reveals significantly elevated arginine levels in plasma and cerebrospinal fluid.  The deficient enzyme in this patient is normally involved in the production of which of the following?

A) γ-aminobutyric acid
B) Glutamine
C) Homocysteine
D) Orotic acid
E) Serotonin
F) Urea

Correct Answer:

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