A 32-year-old woman from rural India presents with a 3-week history of tea-colored urine, facial puffiness, and progressive dyspnea. She denies recent infections. On examination, blood pressure is 168/104 mmHg, respiratory rate is 28/min, and bilateral fine crackles are heard. Urinalysis shows 4+ protein, 3+ blood, RBC casts, and WBC casts. Serum creatinine is 2.4 mg/dL (baseline 0.8 mg/dL), and serum albumin is 3.2 g/dL. Serum C3 level is 45 mg/dL (normal 80–160). C4 is normal. ASO titer is markedly elevated. Renal ultrasound shows normal-sized kidneys with increased echogenicity. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Post-Streptococcal Glomerulonephritis (PSGN) with Acute Kidney Injury This patient presents with **acute nephritic syndrome** superimposed on **acute kidney injury (AKI)**, consistent with severe PSGN. The key diagnostic clues are: ### Pivotal Clinical Features **High-Yield:** The combination of: 1. **Elevated ASO titer** (streptococcal antigen exposure) 2. **Isolated C3 depression** (C4 normal) — classic for PSGN 3. **Acute nephritic syndrome** (hematuria, RBC casts, hypertension, edema) 4. **Rapid rise in creatinine** (0.8 → 2.4 mg/dL in 3 weeks) 5. **Normal-sized kidneys** (excludes chronic glomerulonephritis) **Key Point:** PSGN can present with severe manifestations including pulmonary edema, hypertensive encephalopathy, and AKI requiring dialysis in 5–10% of cases, particularly in adults. ### Pathophysiology of Severe PSGN ```mermaid flowchart TD A[Group A Streptococcal Infection]:::outcome --> B[Immune Complex Formation]:::action B --> C[Glomerular Basement Membrane Deposition]:::action C --> D[Complement Activation via Classical Pathway]:::action D --> E[C3 Cleavage & Deposition]:::action E --> F{Magnitude of Inflammation}:::decision F -->|Mild-Moderate| G[Self-limited Nephritic Syndrome]:::outcome F -->|Severe| H[Acute Proliferative GN + AKI]:::urgent H --> I[Pulmonary Edema, Hypertensive Crisis]:::urgent I --> J[Possible Need for Dialysis]:::action ``` ### Laboratory Interpretation | Parameter | Finding | Interpretation | | --- | --- | --- | | **C3 level** | 45 mg/dL (LOW) | Complement consumption by immune complexes | | **C4 level** | Normal | Rules out systemic lupus or cryoglobulinemia | | **ASO titer** | Markedly elevated | Evidence of recent/current GAS infection | | **RBC casts** | Present | Glomerular origin of hematuria | | **WBC casts** | Present | Active glomerular inflammation | | **Proteinuria** | 4+ | Both nephritic (RBC casts) and nephrotic (proteinuria, hypoalbuminemia) features | | **Creatinine** | 2.4 mg/dL | Significant reduction in GFR (AKI) | | **Kidney size** | Normal | Acute process, not chronic | **Clinical Pearl:** The presence of **both RBC casts AND WBC casts** indicates active proliferative glomerulonephritis with inflammatory cell infiltration. ### Why This Is PSGN (Not RPGN) **Mnemonic: PSGN vs RPGN — "COMP vs ANCA"** - **PSGN:** **C**omplement-mediated, **O**lder children/young adults, **M**ild-to-moderate proteinuria, **P**reserved C4 - **RPGN:** **A**NCA-positive, **N**ecrotizing vasculitis, **C**rescentic GN, **A**cute progression to ESRD This patient lacks: - ANCA positivity (would be expected in RPGN) - Systemic vasculitis features (sinusitis, pulmonary hemorrhage, rash) - Crescentic pattern on biopsy (though not done here) ### Differential Diagnosis Exclusion | Diagnosis | Why Not This Patient | | --- | --- | | **RPGN (ANCA)** | No systemic vasculitis; C3 depression (not typical of ANCA); elevated ASO titer points to streptococcal etiology | | **Lupus nephritis** | No ANA/anti-dsDNA mentioned; no systemic lupus features; C4 is normal (lupus shows C3 AND C4 depression) | | **Membranous nephropathy** | Presents with nephrotic syndrome (proteinuria >3.5 g/day), NOT nephritic syndrome with hematuria and casts | ### Management of Severe PSGN with AKI 1. **Fluid management:** Restrict to 800–1000 mL/day; diuretics (furosemide) if pulmonary edema 2. **Antihypertensives:** ACE inhibitor or ARB (target BP <140/90); avoid NSAIDs (worsen renal function) 3. **Antibiotic:** Penicillin V if active GAS infection (though usually resolved by presentation) 4. **Dialysis:** Indicated if K^+^ >6.5 mEq/L, fluid overload unresponsive to diuretics, or uremia 5. **Immunosuppression:** NOT indicated; PSGN is self-limited **Key Point:** Corticosteroids and immunosuppressive agents do NOT improve outcomes in PSGN and are reserved for rare cases with crescentic transformation (>50% of glomeruli). ### Prognosis - **Children:** >95% achieve complete recovery - **Adults:** 80–90% recover renal function; 10–20% may have persistent proteinuria or mild renal insufficiency - **Poor prognostic factors:** Age >40, delayed presentation, severe AKI at presentation, hypertension This patient's severe presentation warrants close monitoring and possible renal biopsy if clinical course worsens or does not improve within 3–4 weeks.