A 3-year-old male presents with a firm, non-tender abdominal mass. What is the most likely diagnosis?

In a 3-year-old male presenting with a firm, non-tender abdominal mass, the most likely diagnosis is Wilms' tumor, also known as nephroblastoma. This is the most common form of kidney cancer in children, typically occurring between the ages of 2 and 5 years. The mass associated with Wilms' tumor can often be palpated during a physical examination and is usually described as smooth and firm, though non-tender.

Wilms' tumor commonly presents without accompanying pain or significant symptoms, making it identifiable initially as an abdominal mass. Patients may present with other systemic symptoms such as hematuria or hypertension, but the mass itself is characteristically firm and non-tender.

In contrast, conditions like intussusception usually present with acute abdominal pain, vomiting, and potential signs of obstruction, which make a non-tender mass less likely. Rhabdomyosarcoma, while it can present as a mass, is typically associated with pain and other systemic symptoms. Post-streptococcal glomerulonephritis would not typically present as an abdominal mass at all; instead, it is characterized by hematuria, hypertension, and edema following a streptococcal infection.

Thus,