A biopsy report shows Tzanck cells. What is the diagnosis?

Tzanck cells (acantholytic/rounded keratinocytes with ballooning degeneration) are the classic cytologic finding in herpetic infections — most often associated with HSV-1 in the oral cavity (HSV-2 causes the same cytologic finding but is far more often a genital infection). Lichen planus shows a band-like lymphocytic infiltrate and basal cell degeneration, not Tzanck cells, and HPV shows koilocytes, not Tzanck cells.