Antistreptolysin O (ASO) titre is used to support a diagnosis of post-streptococcal rheumatic fever. The biological basis of this test is that antistreptolysin O antibody:

• A Inhibits the action of streptokinase
• B Agglutinates group A streptococci directly
• C Lyses streptococcal cell membranes
• D Neutralises the haemolytic activity of streptolysin O ✓

Explanation

Streptolysin O is an oxygen-labile, thiol-activated haemolysin produced by group D streptococci that lyses red blood cells in the reduced state. Patients infected with these organisms produce anti-streptolysin O antibody, which neutralises this haemolytic activity. In the test, serial serum dilutions are incubated with streptolysin O followed by red cells; inhibition of haemolysis indicates antibody. Streptokinase is a fibrinolytic enzyme measured by a different assay, and ASO does not react with bacterial cells themselves.

Reference: Ananthanarayan and Paniker Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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