A 23-year-old female patient developed bilateral lower extremity edema with reduced urine output 3 days after fever and sore throat. Blood pressure (BP) is 160/100 mmHg, hemoglobin (Hb) 92 g/L, urinary protein (2+), red blood cells (RBC) (+++), granular casts 0-2/HPF, and serum albumin (Alb) 32 g/L. Which of the following tests is most significant for diagnosis and treatment in this case?