A 30 year old pregnant woman presents to a physician with painful oral ulcers. Physical examination demonstrates widespread erosions of her mucous membranes. Close examination reveals a friable mucosa, but no well-defined aphthous ulcers. Biopsy of perilesional mucosa demonstrates acantholysis; direct immunofluorescence demonstrates an intraepidermal band of IgG and C3. Which of the following is the most likely diagnosis?