Commonest extracranial complication of CSOM: (A) Petrositis, (B) Labyrinthitis, (C) Subperiosteal abscess, (D) Facial nerve paralysis
Answer is C. Pus may break through mastoid coex leading to subperiosteal abscess which may even burst on surface leading to a discharging fistula It occurs as a sequence of mastoiditis which is a most common complication Ref: Dhingra 7e pg 84.