Pemphigus Vulgaris — Nikolsky Positive

A 52-year-old man of Ashkenazi Jewish descent presents with a 3-week history of painful oral erosions and scattered flaccid blisters on his trunk and flexural areas. On examination, gentle lateral pressure applied to clinically normal-appearing skin adjacent to an erosion results in extension of the bulla and formation of a new blister. The finding marked **C** in the diagram is documented as positive. Which of the following best explains the pathophysiology underlying this clinical sign?