This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Skin
U-serrated pattern in direct immunofluorescence is seen in: 
Select and explain. 
A. Epidermolysis bullosa acquisita, B. Bullous pemphigoid, C. Linear IgA disease, D. Dermatitis herpetiformis
A: Epidermolysis bullosa acquisita
Answer: A: Epidermolysis bullosa acquisita 
Explanation: 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.