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What is The most important prognostic factor of Wilms' tumor - and explain in detail?
Anaplastic nuclear change is the only criterion of "unfavourable" histology in Wilm's tumor and all Wilm's tumor lacking this feature are designated as having "favourable histology".
Anaplastic nuclear change reflects extreme polypoidy & is usually apparent under low magnification. 

"Anaplastic histology remains a critical determinant of adverse prognosis. Even anaplasia restricted to kidney confers an increased risk of recurrence & death, underscoring the need for correctly identifying this histologic features." —Robbin's 8th/e pg 481