The nephrotic–nephritic distinction is a fundamental framework. Some conditions overlap both (e.g. MPGN, lupus nephritis class IV). RBC casts on urine microscopy are pathognomonic of glomerulonephritis — their presence always points to a nephritic process.
Class IV (diffuse proliferative) is the most common and most severe form of lupus nephritis. Treatment: induction with MMF or IV cyclophosphamide + high-dose corticosteroids, followed by maintenance MMF or azathioprine. Renal biopsy is required for classification and guides treatment intensity.