**Core Concept**
Leukemia is the most common malignancy in children, accounting for a significant proportion of pediatric cancer cases. This is due to the vulnerability of rapidly dividing cells in children's developing immune systems. Acute lymphoblastic leukemia (ALL) is the most prevalent type, followed by acute myeloid leukemia (AML).

**Why the Correct Answer is Right**
Leukemia arises from the clonal expansion of malignant cells in the bone marrow, leading to impaired hematopoiesis and increased susceptibility to infections. The pathogenesis involves genetic mutations in oncogenes and tumor suppressor genes, such as BCR-ABL1, MLL-AF4, and TP53, which disrupt normal cell cycle regulation. The high incidence of leukemia in children is attributed to the high proliferation rate of hematopoietic stem cells, making them more susceptible to malignant transformation.

**Why Each Wrong Option is Incorrect**
**Option A:** Retinoblastoma is a rare intraocular malignancy primarily affecting young children, but it is not the most common malignancy in this age group.

**Option C:** Wilms tumor, also known as nephroblastoma, is a common renal malignancy in children, but its incidence is significantly lower than that of leukemia.

**Option D:** Neuroblastoma is a pediatric extracranial solid tumor, but it is less common than leukemia in children.

**Clinical Pearl / High-Yield Fact**
The majority of childhood leukemia cases are diagnosed between the ages of 2 and 5 years, with a slight male predominance. Early detection and aggressive treatment have improved the prognosis for children with leukemia, with overall survival rates exceeding 80%.

**✓ Correct Answer: B. Leukemia**