A- Lupus nephritis: This is the most likely diagnosis given the context of SLE (Systemic Lupus Erythematosus) and the presence of nephritis. Lupus nephritis is a common complication of SLE, characterized by inflammation of the kidneys caused by the autoimmune disease. The high proteinuria and presence of RBCs in the urine support this diagnosis, as lupus nephritis can cause significant damage to the kidney's filtering units, leading to protein loss and hematuria.

Membranous nephropathy: This condition is known for causing high levels of proteinuria. While it can be primary (idiopathic), it can also be secondary to other diseases, including SLE. The presence of high proteinuria (+1 on urine analysis, though this seems to be an underestimate given the description of "very high protein in urine") could suggest membranous nephropathy, especially if the biopsy shows characteristic changes.
Focal segmental glomerulosclerosis (FSGS): FSGS can also present with significant proteinuria and can be a cause of nephrotic syndrome. It might be considered, especially if there's suspicion of a secondary form related to the underlying SLE or other factors.

Vasculitis (e.g., ANCA-associated vasculitis): Although less likely given the context of SLE, vasculitis affecting the kidneys could present with hematuria and proteinuria. Missing this diagnosis could lead to significant morbidity and mortality due to the potential for rapid progression to kidney failure and the need for specific immunosuppressive treatment.
Anti-glomerular basement membrane (GBM) disease: This rare autoimmune disorder can cause rapidly progressive glomerulonephritis and presents with hematuria and renal failure. It's crucial to consider and rule out due to its aggressive nature and the need for prompt treatment.

Membranoproliferative glomerulonephritis (MPGN): While MPGN can be associated with SLE and presents with hematuria and proteinuria, the normal complement levels make this less likely, as MPGN is often associated with complement activation and consumption. However, it remains a consideration in the differential diagnosis, especially if there are other suggestive features on biopsy.
IgA nephropathy: Although IgA nephropathy is a common cause of hematuria, it typically presents with gross hematuria during or immediately following a respiratory infection, which is not mentioned in the scenario. However, it could be considered in a broad differential diagnosis for hematuria and proteinuria, especially if SLE nephritis is not confirmed.