Which of the following is NOT a streptococcal extracellular product?

Isomerase is indeed the correct choice as it is not classified as a streptococcal extracellular product. The other options listed—streptokinase, streptolysin S, and streptolysin O—are all well-known extracellular products produced by streptococci, which play significant roles in the pathogenesis of infections caused by these bacteria.

Streptokinase is an enzyme that facilitates the breakdown of blood clots, allowing the bacteria to spread through tissues. Streptolysin S and streptolysin O are two hemolysins that can lyse red blood cells, compromising the host's immune response and increasing the virulence of streptococcal infections.

By distinguishing isomerase from these well-documented extracellular products, it highlights the specific participation of enzymes and toxins in the biological activity and pathogenicity of streptococci.