What solid renal mass is most characteristic in a 2-year-old boy presenting with hematuria and a palpable flank mass?

Wilm's tumor is the most common type of kidney cancer in children, particularly in those around the age of 2, which makes it highly characteristic for this age group. It is a type of nephroblastoma that typically presents with symptoms such as hematuria (blood in the urine) and a palpable flank mass, as seen in this clinical scenario.

This tumor usually arises from the embryonic kidney tissue and is often detected in early childhood. Due to its prevalence and the distinct age range, along with the associated symptoms, Wilm's tumor stands out as the most likely diagnosis.

In contrast, other conditions like hypernephroma (clear cell carcinoma) primarily occur in adults rather than young children, making it unlikely in this scenario. Neuroblastoma, while it can occur in young children, typically arises from adrenal gland tissues or sympathetic nervous tissues, presenting with different symptoms and locations. Infantile polycystic kidney disease is a genetic disorder characterized by cyst formation in the kidneys and is not typically associated with a solid mass or acute presentation as noted here.

Thus, the combination of the patient's age and presenting symptoms strongly indicates that Wilm's tumor is the most characteristic solid renal mass in this context.