The palate and maxillary ridge of a 56-year-old woman contain multiple flat ulcers of 8 weeks' duration. She has a positive Nikolsky sign and has no evidence of skin or lip lesions. Which of the following should receive serious consideration in the clinical differential diagnosis?

a. Pemphigus vulgaris
b. Herpangina
c. Lichen planus
d. Aphthous stomatitis
e. Herpes simplex infection

A