Q. A 45-year-old man was admitted to the surgical ward after a traffic accident. CT abdomen with contrast reveals multiple splenic lacerations requiring urgent splenectomy. The patient's medical history is significant for allergic rhinitis and chronic giardia infection. He is scheduled for an urgent blood transfusion prior to surgery. Shortly after initiating the blood transfusion, the patient's blood pressures drop, and he complains of difficulty breathing (dyspnea). You order intramuscular epinephrine. Very quickly after receiving the medication, his breathing improves and his blood pressure begins to recover. An immunodeficiency is suspected. Which of the following interleukins is responsible for production of the deficient immune component in this patient?