A 32-year-old Indian woman presents with progressive facial puffiness and lower limb swelling for 3 weeks. She reports foamy urine and has gained 4 kg over this period. On examination: BP 128/82 mmHg, bilateral pitting ankle edema, no ascites. Urine dipstick shows 4+ protein. Serum albumin 2.1 g/dL, total cholesterol 310 mg/dL, creatinine 0.9 mg/dL. Urinary protein excretion is 6.2 g/24 h. Serum complement C3 and C4 are normal. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** In a 32-year-old adult presenting with nephrotic syndrome, normal complement levels, and no hematuria, **Membranous Nephropathy (MN)** is the most common cause of primary nephrotic syndrome in adults worldwide, including in the Indian subcontinent. ### Diagnostic Features Supporting Membranous Nephropathy | Feature | Finding | Significance | |---------|---------|---------------| | Age & sex | 32-year-old woman | MN peaks in adults (30–50 years); can affect women | | Proteinuria | 6.2 g/24 h (nephrotic range) | Classic heavy proteinuria in MN | | Serum albumin | 2.1 g/dL | Severe hypoalbuminemia typical of nephrotic syndrome | | Renal function | Creatinine 0.9 mg/dL | Preserved GFR — consistent with early/primary MN | | Complement levels | C3, C4 normal | **Key differentiator**: MN characteristically has normal complement; MPGN shows low C3/C4 | | Lipid profile | Total cholesterol 310 mg/dL | Secondary hyperlipidemia from ↑ hepatic synthesis | | Edema pattern | Facial + lower limb | Classic nephrotic presentation | **High-Yield:** Membranous nephropathy is the **most common cause of primary nephrotic syndrome in adults** (accounting for ~30–40% of adult nephrotic syndrome cases), as stated in Harrison's Principles of Internal Medicine and Robbins & Cotran Pathologic Basis of Disease. In contrast, Minimal Change Disease (MCD) is the most common cause in **children** (>90%), but accounts for only ~10–15% of adult nephrotic syndrome. ### Why Membranous Nephropathy Fits Best 1. **Adult demographic** — MN is the leading cause of nephrotic syndrome in adults; MCD is predominantly a pediatric disease. 2. **Normal complement** — MN characteristically shows normal C3 and C4 (subepithelial immune complex deposits do not activate complement via the classical pathway efficiently). MPGN typically shows low C3 (and sometimes C4), making it less likely here. 3. **Preserved renal function** — Early/primary MN often presents with preserved GFR; FSGS and MPGN more commonly show renal impairment. 4. **Pathology** — MN shows subepithelial immune complex deposits ("spike and dome" pattern on silver stain) on light microscopy, with thickened GBM; electron microscopy confirms subepithelial deposits. Anti-PLA2R antibodies are positive in ~70–80% of primary MN cases (Harrison's, 21st ed.). ### Why Other Options Are Less Likely - **MPGN (B):** Typically presents with low C3 (complement consumption); often associated with hematuria and RBC casts — absent here. - **FSGS (C):** More common in African Americans; often associated with some degree of renal impairment and hypertension; less likely as the primary diagnosis in this demographic without risk factors. - **Minimal Change Disease (D):** Most common in children; accounts for only ~10–15% of adult nephrotic syndrome. While it can occur in adults, it is statistically less likely than MN in this age group. **Clinical Pearl:** According to Harrison's Principles of Internal Medicine (21st ed.), membranous nephropathy is the single most common cause of idiopathic nephrotic syndrome in adults. The anti-PLA2R antibody test has revolutionized non-invasive diagnosis of primary MN. Kidney biopsy remains the gold standard, showing characteristic "spike and dome" pattern on Jones silver stain. **Mnemonic: MN in Adults = Most Notable cause of Nephrotic syndrome in Adults** — remember MN when an adult presents with nephrotic syndrome and normal complement levels.