'Bulla spread sign' is seen in :

In the traditional "bulla spread" sign or Lutz sign, the margin of an intact bullae is first marked by a pen. Slow, careful and unidirectional pressure applied by a finger to the bulla causes peripheral extension of the bulla beyond the marked margin. The bulla thus extended has an irregular angulated border in pemphigus vulgaris, while a regular rounded border is observed in bullous pemphigoid or other subepidermal blistering disorders. Ref :Longo D.L., Fauci A.S., Kasper D.L., Hauser S.L., Jameson J.L., Loscalzo J. (2012). Chapter 54. Immunologically Mediated Skin Diseases. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.