A 6-month-old presents with recurrent cellulitis and bacteremias due to Staphylococcus aureus. The white blood cell count is 2500 with 5% neutrophils, 10% eosinophils, 35% monocytes, and 50% lymphocytes. The platelet count is 650,000. A brother and a female cousin died at the age of 18 months and 2 years, respectively. The most likely diagnosis is

Correct Answer: C

Rationale: Kostmann disease (severe congenital neutropenia) fits with persistent neutropenia, recurrent Staphylococcus aureus infections, and a family history suggesting autosomal recessive inheritance.

Correct Answer: B

Rationale: Giant granules in neutrophils are a hallmark of Chédiak-Higashi syndrome, visible under light microscopy.

Correct Answer: B

Rationale: Naïve CD8 lymphocytes require activation by antigen-presenting cells and differentiation into cytotoxic T cells, making them the last to act compared to innate cells like NK cells, macrophages, and intraepithelial lymphocytes.

Correct Answer: A

Rationale: Double-stranded viral RNA is a PAMP recognized by innate immune receptors like TLR3, whereas HMGb1 is a DAMP (damage-associated molecular pattern).

Correct Answer: D

Rationale: IL-1 increases vascular permeability to aid WBC migration, not decreases it.