Final answer:
Streptolysin O is the most commonly used serological marker for recent streptococcal infections and is identified through serological testing like agglutination assays.
Step-by-step explanation:
Streptolysin O is the most frequently used serologic indicator of an antibody response to a recent streptococcal infection. It is one of the extracellular enzymes produced by Streptococcus pyogenes, also known as group A streptococcus (GAS). These enzymes, including streptolysin O and S, are key in the pathogenesis of diseases caused by GAS and elicit a strong immune response from the host. Streptolysin O is oxygen-labile, meaning it is inactivated in the presence of oxygen, unlike streptolysin S which is oxygen-stable. The detection of antibodies against streptolysin O in a patient's blood is a crucial diagnostic indicator for recent infection with group A streptococci, which is often identified through serological testing such as an agglutination assay.