A wound culture grows gram-positive cocci in clusters that are catalase-positive and coagulase-positive and ferment mannitol. Which virulence factor binds the Fc portion of IgG?

• A. Hyaluronidase
• B. Elastase
• C. Protein A
• D. Coagulase

Answer: (C)

Staphylococcus aureus evades immune clearance by using Protein A on its surface to bind the Fc region of IgG. This orientation of antibodies prevents proper recognition by Fc receptors on phagocytes and impairs complement-mediated opsonization, making phagocytosis less efficient. The clinical clues—gram-positive cocci in clusters that are catalase-positive, coagulase-positive, and ferment mannitol—fit S. aureus, and Protein A is the virulence factor that specifically binds the Fc portion of IgG. Other factors like hyaluronidase (spreads through tissue), elastase (degrades elastin), or coagulase (clots plasma to wall off bacteria) contribute to virulence in different ways, but they do not bind the Fc region of IgG.