A 58-year-old man presents with gross hematuria and right flank pain for 3 weeks. On examination, a palpable mass is noted in the right flank. Contrast-enhanced CT abdomen shows a 6 cm heterogeneous mass in the right kidney with invasion into the renal vein. Serum creatinine is 1.2 mg/dL, hemoglobin 9.8 g/dL, and alkaline phosphatase is elevated at 120 U/L despite normal liver function tests. What is the most likely diagnosis and what is the pathological grade of this tumor?