<prompt> The following are features of polycythemia rubra vera, except -Increased red cell massLow aerial oxygen saturationPresence of JAK2 mutationSplenomegaly  <response>: In PV, the major pathogenetic mechanism is a tyrosine kinase JAK2 mutation which removes the autoinhibitory control activates the kinases.Due to erythrocytosis, red cell mass will be increased .Splenomegaly produces abdominal fullness.But in PV, oxygen saturation will be high.Reference :Harsh mohan textbook of pathology sixth edition pg no 359. 