A 63-year-old man complains of fever and multiple painful rashes in his oral cavity and on his face and torso. 3-4 days before the rashes appeared he had noticed a burning sensation followed by sharp shooting pain resembling that which occurs during lumbago. The patient takes cytotoxic drugs for leukemia treatment. On examination there are multiple aphthae located in a row on the vermillion border and labial, lingual, and buccal mucosa on the right. The aphthae have hyperemic borders, they are not fused together and are sharply painful on palpation. The right side of the face presents with erythematous spots, vesicles, and erosions. Make the provisional diagnosis: