Review Question - QID 211082

A 16-year-old girl undergoes an emergent appendectomy after presenting to the emergency department with appendicitis. She is given a mixture of nitrous oxide and sevoflurane for induction of anesthesia and intubated for a secure airway during surgery. A few minutes after induction, she is found to have increased end-tidal carbon dioxide and tachycardia. Furthermore, the surgeon notices that her abdomen is extremely rigid before making his incision. Finally, she is found to have hyperkalemia despite having normal serum potassium during preoperative laboratory studies. Defective function of which of the following proteins is most likely responsible for this patient's findings?