Q. A 23-year-old woman comes to the physician’s office for excessive bleeding gums while brushing her teeth for the past week. She also complains of associated pain and it is worse on chewing and swallowing food. Her menstrual history is regular with normal flow. She is a nonsmoker, nonalcoholic, and not taking any drugs. Her temperature is 98.6°F (37°C), pulse is 86/min, blood pressure is 110/80 mmHg, and respirations are 12/min. Physical examination shows a few flaccid blisters on her neck on normal-appearing skin. When enquired, she says that they may be due to facial peels that she used recently. No similar lesions are noted on the rest of the body surface. Intraoral examination shows multiple ulcerations on the gingiva, palatine mucosa, and the floor of the mouth. Ulcers have well-defined borders with whitish-yellow slough on an erythematous base. They are tender and not attached to underlying structures. Biopsy from the lesion shows intraepithelial cleavage with suprabasal acantholysis and retained basal keratinocytes giving an appearance of a “row of tombstones” along the basement membrane. Based on these findings, what is the most likely diagnosis in this patient?