**Core Concept**
Wilms' tumor, also known as nephroblastoma, is a type of **cancer** that primarily affects **children** and is the most common type of **renal cancer** in this age group. It arises from the **metanephric blastema** of the **kidney**. The presentation of Wilms' tumor can vary, but it often involves **abdominal symptoms**.

**Why the Correct Answer is Right**
The most common presentation of a child with Wilms' tumor is a **painless abdominal mass**, which is typically discovered by a parent while bathing the child or during a routine medical examination. This is because Wilms' tumor can grow quite large before causing significant symptoms, and the mass can be felt in the **abdomen**. Other symptoms may include **abdominal pain**, **fever**, and **hematuria**, but these are less common as initial presentations.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because, although abdominal pain can be a symptom of Wilms' tumor, it is not the most common presentation.
**Option B:** and **Option D:** are incorrect for similar reasons, as they are not the most typical initial symptoms.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Wilms' tumor often presents as a **painless abdominal mass**, and any child with an **unexplained abdominal mass** should be evaluated for this condition.

**Correct Answer:** D. A painless abdominal mass.