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?

This is pemphigus vulgaris, in which autoantibody directed against transmembrane cadherin adhesion molecules induced acantholysis (breakdown of epithelial cell-cell connections) with resulting intraepidermal blister formation. It may develop spontaneously or following triggers such as drugs (thiols, penicillamine), physical injury (burns), cancer, pregnancy, other skin diseases, and emotional stress. Pemphigus vulgaris is a relatively rare blistering disease; it is seen more commonly in patients with Jewish or Mediterranean heritage. In addition to the usually prominent oral ulcers, uncomfoable skin erosions can also occur when the blisters rupture rapidly and are not observed. The epidermis at the edge of these erosions is often easily disrupted by sliding pressure (Nikolsky sign). Bullous pemphigoid is characterized by deeper blisters, occurring at the dermal-epidermal junction. Dermatitis herpetiformis is characterized by severe, intense pruritus and groups of papules and vesicles. Herpes simplex I or II can show multinucleated giant cells on scrapings of the ulcer base. Ref: Suurmond D. (2009). Section 34. Disorders of the Mouth. In D. Suurmond (Ed), Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology, 6e.