**Core Concept**
Kostmann's Syndrome is a rare congenital disorder characterized by severe neutropenia, leading to recurrent infections and increased mortality. It is caused by mutations in the ELA2 gene, which encodes the neutrophil elastase enzyme essential for neutrophil function.

**Why the Correct Answer is Right**
The correct answer is G-CSF (Granulocyte-Colony Stimulating Factor) because it is the most effective treatment for Kostmann's Syndrome. G-CSF stimulates the production of neutrophils in the bone marrow and increases their release into the circulation, thereby correcting the neutropenia. This leads to improved infection control and reduced mortality. G-CSF works by binding to its receptor on the surface of hematopoietic cells, activating the JAK/STAT signaling pathway, and upregulating the expression of genes involved in neutrophil production.

**Why Each Wrong Option is Incorrect**
**Option A:** Corticosteroids are not effective in treating Kostmann's Syndrome as they do not address the underlying cause of neutropenia.

**Option B:** Antimicrobial prophylaxis is a supportive measure, but it is not a curative treatment for Kostmann's Syndrome.

**Option C:** Bone marrow transplantation may be considered in severe cases, but it is not the first line of treatment and carries significant risks.

**Clinical Pearl / High-Yield Fact**
G-CSF is a recombinant protein that mimics the action of the natural human G-CSF, which is essential for the development and maturation of neutrophils.

**Correct Answer: B. G-CSF**