A 3-month-old infant has suffered recurrent episodes of fever, skin pustules, and pneumonia since the first week of life. Repeated blood counts have shown absolute neutrophil count (ANC) to be less than 500/mm3, whereas hemoglobin and platelet count are normal. The most likely diagnosis is
**Core Concept**
Severe congenital neutropenia, also known as Kostmann syndrome, is a rare genetic disorder characterized by a profound and early onset of neutropenia, leading to recurrent infections and increased susceptibility to infections.
**Why the Correct Answer is Right**
The infant's clinical presentation of recurrent fever, skin pustules, and pneumonia with an absolute neutrophil count (ANC) less than 500/mm^3 suggests a severe congenital neutropenia. This condition is caused by mutations in the ELA2 gene, which encodes for neutrophil elastase. Neutrophil elastase is essential for the proper functioning of neutrophils, and its deficiency leads to impaired neutrophil maturation and function. The normal hemoglobin and platelet count in this infant rule out other causes of cytopenias, making severe congenital neutropenia the most likely diagnosis.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not match the clinical presentation and laboratory findings of the infant. Chronic granulomatous disease (CGD) is a different condition characterized by recurrent infections due to impaired phagocytic function, but it typically presents with granulomatous inflammation and not neutropenia.
**Option B:** This option is incorrect as it does not accurately describe the infant's condition. Leukemia is a malignant disorder of the hematopoietic system, which is not suggested by the infant's clinical presentation and laboratory findings.
**Option C:** This option is incorrect as it does not match the infant's clinical presentation and laboratory findings. Cyclic neutropenia is a rare condition characterized by periodic neutropenia, but it typically presents with a regular cycle of neutropenia and not as early onset as seen in this infant.
**Clinical Pearl / High-Yield Fact**
Severe congenital neutropenia is a rare but life-threatening condition that requires early recognition and management to prevent recurrent infections and improve survival.
**Correct Answer: C.**