A 2-year-old child presents with recurrent pyogenic bacterial infections and impaired opsonization. Lab tests reveal normal numbers of B cells and T cells, but a deficiency in the classical and alternative complement pathways. Which complement component is most likely deficient?

The correct answer to the question "A 2-year-old child presents with recurrent pyogenic bacterial infections and impaired opsonization. Lab tests reveal normal numbers of B cells and T cells, but a deficiency in the classical and alternative complement pathways. Which complement component is most likely deficient?" is Option C: C3.

The other options — A) C1, B) C2, D) C4 — are incorrect for this USMLE Exam Prep question. As explained above: C3 is central to both the classical and alternative complement pathways. A deficiency in C3 would impair opsonization (via C3b) and compromise the entire complement cascade, leading to recurrent bact…