Trevor is a 20-year-old male presenting with a history of fatigue, shortness of breath, gallstones, and yellowing of the skin and eyes (jaundice). An examination of his blood reveals his red blood cells have an abnormal elliptical shape which causes premature damage and rupture of the cells. Trevor is diagnosed with a genetic disease, Hereditary elliptocytosis. The disease is caused by mutations in some of the proteins of the membrane skeletal network responsible for the stability of the red blood cell. What organelle is defective in Trevor's red blood cells?