A young woman with recurrent pyogenic infections and low C3 is most likely to have which of the following?

• A. Autoimmune disease with continual antigen-antibody activity causing consumption of C3
• B. DiGeorge syndrome
• C. Decreased production of C3
• D. Inactive form of C2, a precursor of C3

Answer: (C)

C3 is the central player in the complement system and is essential for opsonizing and clearing encapsulated bacteria. When C3 levels are reduced because production is decreased, the body cannot efficiently tag these bacteria for phagocytosis, leading to recurrent pyogenic infections. This direct link between low C3 and impaired opsonization makes a production deficiency the most straightforward explanation for the pattern described.

If C3 were low from ongoing consumption by immune complexes (as might occur with autoimmune disease), you would expect signs of immune complex–mediated pathology in addition to or instead of a pure infection pattern. DiGeorge syndrome involves T-cell deficiency and would present with different infection risks. An inactive form of C2 would impair the classical pathway and can affect C3 activation, but it doesn’t as clearly point to a simple, isolated decrease in C3 production. Therefore, decreased production of C3 best fits the presentation.