A 64-year-old woman with a BP of 162/90 mmHg six months ago (normal urinalysis and renal function) has been on captopril since then. One month ago, she developed polyuria, fatigue, and BP of 190/110 mmHg. UA: protein +, renal function: BUN 16 mmol/L, Cr 324 μmol/L, serum K 3.0 mmol/L. Renal ultrasound: left kidney 11.8 cm × 5.2 cm, right kidney 9.0 cm × 3.8 cm. What is the most appropriate initial treatment?