**Core Concept**
The question is testing the ability to diagnose a renal mass in a pediatric patient based on ultrasound findings. This involves understanding the typical ultrasound characteristics of common renal tumors in children and differentiating between them.

**Why the Correct Answer is Right**
The most likely diagnosis is Wilms tumor, also known as nephroblastoma. This is the most common renal tumor in children, accounting for about 6-7% of all pediatric cancers. Wilms tumor typically presents as a solid, circumscribed hypoechogenic mass on ultrasound. It arises from the metanephric blastema, which is the precursor tissue to the renal parenchyma. The tumor is thought to result from a combination of genetic and environmental factors, including mutations in the WT1 gene.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because neuroblastoma is a different type of tumor that typically presents as a posterior mediastinal mass or a paraspinal mass, rather than a renal mass.

**Option B:** This option is incorrect because congenital mesoblastic nephroma is a rare tumor that typically presents in the neonatal period, rather than at 3 years of age.

**Option C:** This option is incorrect because clear cell sarcoma of the kidney is a rare tumor that typically presents in older children, rather than in a 3-year-old boy.

**Option D:** This option is incorrect because rhabdoid tumor of the kidney is a highly aggressive tumor that typically presents in younger children, rather than in a 3-year-old boy.

**Clinical Pearl / High-Yield Fact**
The triad of abdominal pain, hematuria, and a palpable abdominal mass is classic for Wilms tumor. However, many children with Wilms tumor are asymptomatic and the tumor is detected incidentally on imaging.

**Correct Answer: D. Rhabdoid tumor of the kidney is another possible option, however, given the age and presentation, Wilms tumor is the most likely diagnosis.**