A 68-year-old man presents with tense bullae on the flexural aspects of both forearms and inner thighs for 3 months. Histopathology shows subepidermal bulla with intact basal layer and eosinophilic infiltrate. Direct immunofluorescence reveals linear IgG and C3 deposits along the basement membrane zone. What is the most common site of involvement in bullous pemphigoid?