**Core Concept**
The patient's presentation with painless gross hematuria and a filling defect on excretory urogram suggests a renal mass, likely a renal tumor. The lower renal infundibulum is a common location for renal tumors, especially in children.

**Why the Correct Answer is Right**
The next investigation should be a renal ultrasound to further evaluate the nature and characteristics of the renal mass. Renal ultrasound is a non-invasive, widely available, and cost-effective imaging modality that can provide information on the size, location, and characteristics of the renal mass, such as its echogenicity and vascularity. This information is essential for determining the next steps in management, including the need for biopsy or surgical resection.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is not the next investigation because it exposes the patient to radiation and is more expensive than ultrasound. While CT scan can provide detailed information on the renal mass, it is not the initial imaging modality of choice in this scenario.
**Option B:** MRI is not the next investigation because it is more expensive and less widely available than ultrasound. MRI can provide detailed information on the renal mass, but it is not the initial imaging modality of choice in this scenario.
**Option C:** IVP is not the next investigation because it is less sensitive than ultrasound for detecting renal masses and can expose the patient to radiation. IVP is more useful for evaluating the function of the urinary tract.

**Clinical Pearl / High-Yield Fact**
In children and young adults with painless gross hematuria and a renal mass, the most common diagnosis is Wilms tumor, a type of renal cell carcinoma that is highly curable with early detection and treatment.

**Correct Answer: A. Renal ultrasound. Renal ultrasound is the next investigation to be done.**