A 28-year-old woman presents with progressive swelling of the face and legs over 3 weeks. She reports sudden-onset foamy urine and weight gain of 8 kg. On examination: BP 128/82 mmHg, bilateral pitting pedal edema, ascites. Laboratory findings: serum albumin 2.1 g/dL, total protein 4.8 g/dL, 24-hour urine protein 8.2 g, serum creatinine 0.9 mg/dL, blood urea 28 mg/dL. Lipid profile shows total cholesterol 320 mg/dL, LDL 240 mg/dL. Urinalysis: 4+ proteinuria, few RBCs, no casts. Renal ultrasound: normal-sized kidneys, normal echotexture. Serum complement C3 and C4 levels are normal. Which of the following is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Membranous Nephropathy ### Key Clinical Features Supporting Membranous Nephropathy **Key Point:** Membranous nephropathy (MN) is the most common cause of primary nephrotic syndrome in adults (particularly those aged 20–60 years), accounting for approximately 30–40% of adult nephrotic syndrome cases. It classically presents with a pure nephrotic picture and normal complement levels. ### Diagnostic Criteria Present in This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Age & sex** | 28-year-old woman | MN is common in adults; also seen in young women (consider secondary causes like lupus, HBV) | | **Nephrotic range proteinuria** | 8.2 g/24 h | Confirms nephrotic syndrome (>3.5 g/day) | | **Serum albumin** | 2.1 g/dL | Severe hypoalbuminemia typical of nephrotic syndrome | | **Hyperlipidemia** | Total cholesterol 320 mg/dL, LDL 240 mg/dL | Secondary hyperlipidemia from increased hepatic lipoprotein synthesis | | **Renal function** | Normal (Cr 0.9, BUN 28) | Preserved GFR — MN typically does not cause early renal dysfunction | | **Urinalysis** | 4+ proteinuria, few RBCs, **no casts** | Mild hematuria can occur in MN; absence of RBC casts argues against proliferative GN | | **Complement levels** | C3 and C4 **normal** | Critical finding: MN is a non-complement-consuming disease; normal C3/C4 rules out MPGN and post-infectious GN | | **Ultrasound** | Normal kidneys, normal echotexture | No structural disease | ### Why Membranous Nephropathy Is Most Likely 1. **Adult patient with pure nephrotic syndrome** — MN is the leading cause of nephrotic syndrome in adults, whereas MCD is more common in children and adolescents. 2. **Normal complement levels** — MN does not activate complement via the classical or alternative pathway in a way that depletes C3/C4. MPGN (option C) characteristically shows low C3 ± C4. 3. **Absence of RBC casts and no nephritic features** — argues against proliferative GN (MPGN, IgA nephropathy). 4. **Insidious but progressive course** — 3-week history of worsening edema and foamy urine is consistent with MN's typical subacute presentation. 5. **Mild hematuria (few RBCs)** — can be seen in MN due to subepithelial immune complex deposition causing mild glomerular injury, unlike MCD which is typically non-hematuria. ### Why Other Options Are Less Likely - **Minimal Change Disease (A):** More common in children; in adults >20 years, MN is more prevalent. MCD rarely causes hematuria (even mild). The 3-week progressive course with few RBCs is more consistent with MN. - **FSGS (B):** Can present similarly but is more common in African Americans, patients with obesity, or those with prior renal injury. FSGS is a diagnosis of exclusion on biopsy. - **MPGN (C):** Characteristically presents with **low C3** (and often low C4 in Type I). Normal complement levels effectively exclude MPGN. ### Pathophysiology of Membranous Nephropathy **High-Yield:** Primary MN is caused by autoantibodies (most commonly anti-PLA2R — phospholipase A2 receptor antibodies) targeting podocyte antigens. Immune complex deposition occurs in the **subepithelial space**, leading to GBM thickening ("spike and dome" pattern on electron microscopy and silver stain). This disrupts the slit diaphragm and causes massive proteinuria without significant inflammatory cell infiltration, explaining the normal complement levels and absence of casts. ### Comparison Table: MCD vs FSGS vs MPGN vs Membranous Nephropathy | Feature | MCD | FSGS | MPGN | **Membranous** | |---------|-----|------|------|----------------| | Age | Children | Any | Young adults | **Adults 20–60** | | Complement | Normal | Normal | **Low C3** | **Normal** | | Hematuria | Rare | Occasional | Common | **Mild (few RBCs)** | | Casts | None | Rare | RBC casts | **None/rare** | | LM | Normal | Focal sclerosis | Mesangial proliferation | **GBM thickening** | | EM | Foot process effacement | Foot process effacement | Subendothelial deposits | **Subepithelial deposits** | | Steroid response | Excellent | Variable | Poor | Variable | **Clinical Pearl:** Anti-PLA2R antibody testing (serum) is now the first-line non-invasive test for primary membranous nephropathy, with ~70–80% sensitivity. Renal biopsy remains the gold standard for definitive diagnosis. Secondary causes (lupus, HBV, HCV, malignancy, drugs like NSAIDs/gold) must be excluded, especially in young women. *Reference: Harrison's Principles of Internal Medicine, 21st edition, Chapter on Glomerular Diseases; Robbins & Cotran Pathologic Basis of Disease, 10th edition.*