A 20-year-old male presents with fatigue and poor appetite for 1 month, oliguria, edema, and hypertension (HTN) for 1 week. Lab findings include anemia, hematuria, proteinuria, normal complement C3, elevated serum creatinine (Cr) and blood urea nitrogen (BUN), and bilateral enlarged kidneys on ultrasound. Clinical diagnosis is 'acute kidney injury (AKI).' If serum anti-glomerular basement membrane (GBM) antibody is positive, which of the following is the most likely diagnosis?