Q. A 2-year-old male presents to the emergency department with a history of recurrent infections and easy bruising. The patient's parents report that he has had several ear infections, pneumonia, and skin infections in the past year and has had difficulty recovering from these infections. They also report that he bruises easily, even with minimal trauma. The patient's physical examination reveals several ecchymoses on his face and extremities, as well as petechiae (see image). The patient also has multiple areas of impetigo on his feet and arms. You order a complete blood count, which reveals neutropenia and thrombocytopenia. Further testing, including a peripheral blood smear, reveals small, irregularly-shaped platelets. Based on these findings, you suspect the patient may have a rare, X-linked recessive immunodeficiency disorder that affects the immune system, blood clotting, and the formation of blood cells. To confirm the diagnosis, you order a gene mutation analysis. Based on the information you have at this point in time, a mutation in which of the following genes is the most likely underlying etiology for this patient’s symptoms?