A 20-year-old woman is seen during a follow-up visit for hematuria. She was evaluated 1 week ago for hematuria of 5 days' duration. She recalled having a sore throat around the time of onset of hematuria, but no fever, dysuria, flank pain, or other symptoms. Urinalysis at that time showed too numerous to count erythrocytes/hpf with a few erythrocyte casts. Laboratory studies at that time showed the following: normal complement levels, a serum creatinine level of 0.7 mg/dL (61.9 µmol/L), negative antinuclear antibodies and ANCA, and a urine protein-creatinine ratio of 1100 mg/g. Kidney ultrasound was normal. She has been healthy, and her only medication is an oral contraceptive pill.
On physical examination, temperature is 37.1 °C (98.7 °F), blood pressure is 130/80 mm Hg, pulse rate is 78/min, and respiration rate is 16/min. BMI is 22. Mild pharyngeal congestion is noted. There is no edema. The remainder of the physical examination, including skin, joints, and nasal and oral mucosa, is normal.
Current laboratory studies show a urinalysis with 5-10 erythrocytes/hpf without casts, a serum creatinine level of 0.6 mg/dL (53 µmol/L), and a urine protein-creatinine ratio of 100 mg/g.
Which of the following is the most appropriate next step in management?