Which of the following Streptococcal antigen cross reacts with synovial fluid?: (A) Carbohydrate, (B) Cell wall protein, (C) Capsular hyaluronic acid, (D) Peptidoglycan
Answer is C. Hyaluronan (hyaluronic acid) is a naturally occurring polysaccharide, which is present in extracellular matrices, the synovial fluid of joints, and scaffolding that comprises cartilage.
Hyaluronidase: 

The enzyme is produced by S. pyogenes as well as by other groups like B, C, G streptococci, Streptococcus suis, Streptococcus anginosus, and S. pneumoniae. 
The enzyme splits hyaluronic acid present in host connective tissue, thereby facilitating spread of the bacteria through tissues. The enzyme is antigenic.

Parija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:186