A 40-year-old woman presents with fatigue, dark urine, and swelling in her lower extremities. Urinalysis reveals proteinuria (2+), hematuria with red blood cell casts, and dysmorphic red blood cells. Serum creatinine has rapidly increased from 0.9 mg/dL to 2.2 mg/dL over the past week. Which of the following is the most appropriate next diagnostic step?
✓ Correct Answer: Option C
The patient's presentation of rapidly progressive kidney injury, active urine sediment (red cell casts, dysmorphic RBCs, proteinuria), and edema strongly suggests rapidly progressive glomerulonephritis (RPGN). A renal biopsy is crucial for definitive diagnosis, classification, and guiding specific immunosuppressive therapy.