A 30-year-old male with 1-year history of hypertension (HTN) presents with fever, sore throat for 3 days and gross hematuria for 1 day. Physical exam: BP 160/100 mmHg, pharyngeal erythema, tonsils grade II, no lower limb edema. Laboratory tests: urinary protein 2.56g/d, urine sediment shows RBCs covering entire field, serum creatinine 210 μmol/L, urine specific gravity 1.018, normal liver function, negative hepatitis B serology, elevated blood IgA. No family history of HTN. The most likely pathological diagnosis is: