Trephine histology showing morphologic features that suggest different WHO diagnoses in the same medium- (×400) or high-power (×1000) field. (A) Pyknotic megakaryocytes (indicative of myelofibrosis) in a field otherwise suggestive of true ET (normal cellularity, loose megakaryocyte cluster). (B) Staghorn megakaryocytes in a loose cluster (characteristic of true ET) in a hypercellular marrow (suggesting prefibrotic myelofibrosis). (C) Staghorn megakaryocyte (true ET) immediately adjacent to a cloudlike megakaryocyte (prefibrotic myelofibrosis). (D) Staghorn megakaryocyte (true ET) in a tight cluster (prefibrotic myelofibrosis).