## **Core Concept**
The clinical presentation of painless gross hematuria in a boy, coupled with a filling defect seen on an excretory urogram, suggests a possible renal or urinary tract tumor. The location and description of the filling defect towards the lower renal infundibulum raise suspicion for a specific type of tumor.

## **Why the Correct Answer is Right**
The next best investigation would be one that provides more detailed information about the nature of the filling defect, ideally through direct visualization or tissue diagnosis. Given the suspicion of a tumor, especially in a young patient, **CT-guided biopsy** or a similar diagnostic approach would be crucial. However, in pediatric cases, especially when Wilms tumor (a common renal tumor in children) is a consideration, **Ultrasound-guided biopsy** or **CT-guided biopsy** could be options but the question seems to steer towards a non-invasive imaging that can provide detailed information about the mass and its possible spread.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While an ultrasound can provide information about the renal mass, it might not offer the detailed evaluation of the tumor's extent and its relation to surrounding structures as effectively as a CT or MRI.
- **Option B:** This option seems to suggest a procedure but isn't explicitly stated; however, if implying a more invasive procedure without imaging, it would not be the immediate next step.
- **Option C:** A **Bone scan** might be useful in the staging of certain tumors but would not be the immediate next investigation for characterizing a renal mass.
- **Option D:** **MRI** could be a correct next step for characterizing the renal mass, especially in pediatric patients to avoid radiation, but given the context, **CT abdomen** seems a plausible immediate next step for its quickness and detail in evaluating abdominal structures.

## **Clinical Pearl / High-Yield Fact**
In pediatric patients presenting with painless hematuria and a renal mass, **Wilms tumor** is a critical differential diagnosis. A key point to remember is that **nephroblastoma (Wilms tumor)** often presents with a classic triad of abdominal mass, abdominal pain, and hematuria, though not all features are present in every case. Early and accurate staging is crucial for management.

## **Correct Answer:** .