A 3-year-old male child is brought by his father with complaint of recurrent infections despite proper treatment and hygiene. While reviewing his history, pediatrician notices that the child has been infected multiple times with S. aureus, Pseudomonas & E. coli. Which of the following test will be useful to diagnose condition of the child?

**Core Concept**
The child's recurrent infections, particularly with multiple organisms such as S. aureus, Pseudomonas, and E. coli, suggest an underlying immunodeficiency. This condition may be due to a defect in the immune system's ability to recognize and respond to pathogens.

**Why the Correct Answer is Right**
The child's symptoms are suggestive of Hyper-IgM syndrome, a primary immunodeficiency characterized by impaired class-switching of IgM to other immunoglobulin isotypes. This leads to elevated levels of IgM and impaired responses to infections. The diagnosis of Hyper-IgM syndrome can be confirmed by measuring IgG, IgA, and IgE levels and assessing the child's ability to undergo class-switching.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood culture would be useful in identifying the causative organism in the current infection but would not provide information on the underlying immunodeficiency.
**Option B:** Complete Blood Count (CBC) may show an elevated white blood cell count in response to infection but would not diagnose immunodeficiency.
**Option C:** PCR (Polymerase Chain Reaction) is a useful diagnostic tool for detecting specific pathogens but would not identify the underlying cause of recurrent infections.

**Clinical Pearl / High-Yield Fact**
Hyper-IgM syndrome is a rare primary immunodeficiency characterized by elevated IgM levels and impaired class-switching, leading to recurrent infections.

**Correct Answer:** C.

✓ Correct Answer: A. Negative nitroblue-tetrazolium test