Hereditary Spherocytosis (SPTA1/SPTB, AD)

A 7-year-old boy presents with intermittent jaundice, fatigue, and splenomegaly. His mother underwent cholecystectomy for pigment gallstones at age 22. Laboratory findings show hemoglobin 9.0 g/dL, MCHC 37 g/dL, reticulocytosis 8%, and spherocytes on peripheral smear with a negative direct antiglobulin test. Eosin-5-maleimide (EMA) binding by flow cytometry is decreased. Genetic testing reveals a heterozygous mutation on the structure marked **A** in the diagram. Which of the following best explains the molecular basis of this patient's hereditary spherocytosis?