ASO titre is useful in diagnosis of?

ANSWER: (B) S. PyogenesREF: Jawetz Microbiology, 24th Ed, Ch. 15Also see Medicine 2013 Session 2 (similar question on ASO titre)STREPTOCOCCAL HEMOLYSINS:The Beta-hemolytic group A, S pyogenes elaborates two hemolysins (streptolysins).Streptolysin O is a protein (MW 60,000) that is hemolytically active in the reduced state (available -SH groups) but rapidly inactivated in the presence of oxygen.Streptolysin 0 is responsible for some of the hemolysis seen when growth is in cuts deep into the medium in blood agar plates.It combines quantitatively with antistreptolysin O, an antibody that appears in humans following infection with any streptococci that produce streptolysin O. This antibody blocks hemolysis by streptolysin 0.This phenomenon forms the basis of a quantitative test for the antibody. An antistreptolysin O (ASO) serum titer in excess of160-200 units is considered abnormally high and suggests either recent infection with S pyogenes or persistently high antibody levels due to an exaggerated immune response to an earlier exposure in a hypersensitive person.Streptolysin S is the agent responsible for the hemolytic zones around streptococcal colonies growing on the surface of blood agar plates. It is elaborated in the presence of serum--hence the name streptolysin S. It is not antigenic, but it may be inhibited by a nonspecific inhibitor that is frequently present in the sera of humans and animals and is independent of past experience with streptococci.