* Complex media contain at least one ingredient that is not chemically defined.

* Synthetic media are composed entirely of known chemical components.

MSA contains beef extract and peptone, which are complex mixtures of amino acids, peptides, and other organic compounds. These ingredients are not chemically defined, making MSA a complex medium.

Even though other components like mannitol, NaCl, and agar are chemically defined, the presence of beef extract and peptone makes the overall medium complex.