Review Question - QID 108457

A 14-year-old boy is brought to the emergency department for confusion and extreme fatigue since returning home from school today. His mother is also concerned because his personality has changed over the last 6 months with an increase in aggression and irritability. Further history reveals that he has been suffering from frequent nosebleeds as well as more bleeding than expected from minor trauma. Physical exam reveals splenomegaly and the finding shown in Figure A. Labs show an elevated serum ammonia level. A mutation in which of the following proteins is most likely to be responsible for this patient's symptoms?