**Core Concept**
Wilms tumor, also known as nephroblastoma, is a type of kidney cancer that primarily affects children. It is a malignant tumor of the renal parenchyma, characterized by the presence of embryonic cells.

**Why the Correct Answer is Right**
Wilms tumor commonly presents in children due to the abnormal proliferation of renal blastema cells, which fail to undergo normal differentiation and maturation. This leads to the formation of a solid tumor in the kidney, often accompanied by hemihypertrophy or other congenital anomalies. The tumor typically arises in the lower pole of the kidney and may extend into the renal pelvis or surrounding tissues.

**Why Each Wrong Option is Incorrect**
**Option A:**
The most common presentation of Wilms tumor is not through a palpable abdominal mass, as this is more typical of neuroblastoma or other abdominal tumors.

**Option B:**
While hematuria may be present in some cases, it is not the most common presentation of Wilms tumor.

**Option C:**
Fever is not a characteristic symptom of Wilms tumor, and its presence would suggest an alternative diagnosis.

**Option D:**
Abdominal pain is a non-specific symptom that can be associated with various conditions, including Wilms tumor, but it is not the most common initial presentation.

**Clinical Pearl / High-Yield Fact**
The triad of abdominal mass, hemihypertrophy, and hypertension is often referred to as the "classic triad" of Wilms tumor, although not all patients will exhibit all three features.

**Correct Answer: B. Hematuria**
Hematuria is a common symptom of Wilms tumor, although it may not be present in all cases.