## Core Concept
The question tests knowledge of pediatric oncology, specifically focusing on the most common malignant tumor occurring in infancy. Infancy is a critical period for the development of various tumors, and understanding the epidemiology of these conditions is essential for diagnosis and management.

## Why the Correct Answer is Right
Neuroblastoma is recognized as the most common extracranial solid tumor in children and notably, it is the most common malignant tumor of infancy. It originates from the neural crest cells, which are precursors to the sympathetic nervous system. Neuroblastoma can present at birth or within the first few months of life, often discovered as an abdominal mass. Its clinical presentation varies widely, from asymptomatic to systemic symptoms, depending on the stage and location of the tumor.

## Why Each Wrong Option is Incorrect
- **Option A:** While Wilms tumor (nephroblastoma) is a common renal tumor in children, it typically presents a bit later than infancy, usually around 3-4 years of age.
- **Option B:** Leukemia is a significant concern in pediatric oncology, but it is more common in slightly older children, with the peak incidence around 2-5 years of age.
- **Option C:** This option might refer to another type of tumor, but without specifics, it's challenging to address directly. However, given the context, other tumors like retinoblastoma or hepatoblastoma are less common in infancy compared to neuroblastoma.
- **Option D:** Similar to Option C, without specifics, it's hard to directly refute, but given the context, these are less likely to be the most common malignant tumor of infancy.

## Clinical Pearl / High-Yield Fact
A key point to remember is that neuroblastoma can sometimes regress spontaneously, especially in infants, a phenomenon known as "maturation" or "spontaneous regression." This characteristic is unique and can influence management decisions, particularly in young infants with low-stage disease.

## Correct Answer: D. Neuroblastoma.