U-serrated pattern in direct immunofluorescence is seen in:

Epidermolysis bullosa acquisita: Direct immunofluorescence shows, linear IgG, Ig A, Ig M at the basement-membrane zone with diagnostic U-serrated pattern. Patients' skin split through the lamina lucida with 1 mol/L salt demonstrates the IgG antibodies to be bound to the dermal aspect of the blister. N serrated pattern is seen in bullous pemphigoid. Ref: Rook's textbook of dermatology, 8th edition, Pg 40.53.