## **Core Concept**
The question revolves around a child with recurrent pyogenic infections caused by bacteria with polysaccharide-rich capsules, which hints at an immune deficiency, particularly related to the humoral or B cell-mediated immunity. The key issue here seems to be the inability to effectively combat encapsulated bacteria.

## **Why the Correct Answer is Right**
The correct answer, , involves assessing the child's ability to produce antibodies or to assess B cell function. This is because encapsulated bacteria, such as *Streptococcus pneumoniae*, *Haemophilus influenzae* type b, and *Neisseria meningitidis*, are typically opsonized by antibodies (or complement) before being phagocytosed. A defect in antibody production or function would make it difficult for the child to clear these infections. Testing for serum immunoglobulin levels or specific antibody responses would help diagnose conditions like primary antibody deficiencies.

## **Why Each Wrong Option is Incorrect**
- **Option A:** - This option does not directly relate to assessing the immune response to encapsulated bacteria or common screening tests for recurrent infections.
- **Option B:** - While assessing complement function could be relevant in some cases of recurrent infections (especially with *Neisseria* species), it's less directly related to infections with polysaccharide-rich capsules compared to assessing antibody function.
- **Option C:** - This option might relate to certain types of immune deficiencies but is not as directly relevant to the specific scenario of recurrent infections with encapsulated bacteria.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that patients with recurrent infections, especially those caused by encapsulated bacteria, should be evaluated for primary immunodeficiency diseases, such as common variable immunodeficiency (CVID) or IgA deficiency. A simple and useful initial test in such cases is measuring serum immunoglobulin levels (IgG, IgA, IgM).

## **Correct Answer: B. **