A 50-year-old man is found to have blood in his urine (hematuria) during a routine check up. Vital signs are normal. An owing abdominal CT scan reveals a 2-cm right renal mass, and a CT-guided needle biopsy establishes a diagnosis of renal cell carcinoma. You inform the patient that "staging" of his tumor is key to selecting treatment and evaluating prognosis. Which of the following is the most important staging factor for this patient?