A 20-year-old male presents with oliguria and edema 2 weeks after an upper respiratory tract infection (URI). Physical exam: BP 173/105 mmHg, marked eyelid and subcutaneous edema, fine crackles heard at the lung bases. Laboratory tests: urine protein (++), RBC 20-25/HPF, Hb 110g/L, serum Cr 720 μmol/L, serum K 7.0 mmol/L, CO2CP 18 mmol/L, low C3, mild arteriovenous crossing sign on fundoscopy. The most likely diagnosis is: