Final answer:
Sabouraud's Dextrose Agar (SDA) is not a pathogen but a medium for growing fungi, and fungi generally have lower virulence compared to the bacterial pathogens listed which can lead to severe infections and skeletal tissue discard.
Step-by-step explanation:
The pathogen which is NOT considered to be highly virulent and necessitating skeletal tissue discard is Sabouraud's Dextrose Agar (SDA). SDA is actually a type of growth medium used for cultivating fungi; it isn't a pathogen itself. Moreover, while Fungi (Yeast/Molds) can be pathogenic, they generally have a lower virulence compared to certain bacterial pathogens, and do not always lead to tissue discard unless the infection is severe and systemic.
Pathogens known to cause severe infections and possibly result in skeletal tissue discard include Staphylococcus aureus, Enterococcus spp., Gram Negative Bacilli, and Streptococcus pyogenes. Each of these microorganisms can be highly virulent and are well-documented causes of serious bone and soft tissue infections which might necessitate tissue removal to control the infection.