A 28-year-old man with a history of recurrent sinopulmonary infections presents with fever, productive cough, and chest imaging showing bronchiectasis. Immunoglobulin quantitation reveals: IgG 450 mg/dL (normal 700–1600), IgA <5 mg/dL (normal 70–400), IgM 35 mg/dL (normal 40–230), IgE 8 IU/mL (normal <150). Lymph node biopsy shows preserved architecture with normal germinal centres. Which immunoglobulin deficiency is most likely responsible for this patient's recurrent infections and bronchiectasis?