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Summary Table

Domain	Detail
Risk Factors	Children aged 3 to 12 years (peak age 6 to 7); male predominance (~2:1); recent Group A Streptococcus (GAS) infection of the pharynx or skin; overcrowded living conditions; low socioeconomic settings; tropical climates (favoring impetigo-associated PSGN)
Etiology	Immune complex-mediated glomerulonephritis triggered by nephritogenic strains of Group A beta-hemolytic Streptococcus (notably M protein types 12 and 49)
Patient Presentation	A child presenting 1 to 3 weeks after pharyngitis or 3 to 6 weeks after impetigo with sudden onset of periorbital or facial edema, dark "cola-colored" or "tea-colored" urine, decreased urine output, and mild to moderate hypertension
Classic Physical Exam	Periorbital (facial) edema worse in the morning, peripheral edema, hypertension, signs of fluid overload (pulmonary crackles, jugular venous distention in severe cases); prior skin lesions (honey-crusted impetigo scars) or pharyngeal erythema may still be visible
Key Diagnostic Results	Urinalysis: hematuria with dysmorphic red blood cells (RBCs) and RBC casts; proteinuria (usually subnephrotic); elevated ASO titer (post-pharyngitis) or anti-DNase B (post-impetigo); low serum C3 complement (normalizes within 6 to 8 weeks); elevated BUN and creatinine
Management	Supportive care: sodium and fluid restriction, loop diuretics (furosemide) for edema and hypertension, antihypertensives if needed; antibiotics to eradicate any residual streptococcal infection (penicillin V or amoxicillin); no role for immunosuppression
Keywords	"Cola-colored urine," "periorbital edema," "low C3," "RBC casts," "recent sore throat or impetigo," "nephritic syndrome in a child," "subepithelial humps on electron microscopy"

1. Pathophysiology

PSGN is the prototype of nephritic syndrome and serves as one of the most commonly tested examples of immune complex-mediated glomerulonephritis. Understanding its mechanism is essential because the pathophysiology directly explains every symptom the patient develops.

The disease begins with an infection by a nephritogenic strain of Group A Streptococcus. Not all streptococcal strains cause glomerulonephritis; only those expressing certain virulence factors (such as nephritis-associated plasmin receptor [NAPlr] and streptococcal pyrogenic exotoxin B [SPE B]) are capable of triggering the renal immune response. The latent period between infection and renal disease is important: 1 to 3 weeks after pharyngitis and 3 to 6 weeks after skin infection (impetigo). This delay reflects the time required for antibody production and immune complex formation.

During the infection, the host immune system generates antibodies against streptococcal antigens. These antibodies form circulating immune complexes (antigen-antibody complexes) that deposit in the glomerular basement membrane (GBM), predominantly in the subepithelial space. Additionally, streptococcal antigens may "plant" themselves in the glomerulus first, with antibodies binding to them in situ. Both mechanisms activate the classical complement pathway, leading to consumption of C3 (which is why serum C3 is low). The complement activation recruits neutrophils and monocytes, which infiltrate the glomerulus, causing endocapillary proliferative glomerulonephritis.

The resulting glomerular inflammation has several downstream consequences:

Reduced glomerular filtration rate (GFR): Inflammatory cell infiltration and mesangial proliferation compress and obstruct the glomerular capillary lumen. The drop in GFR leads to oliguria, azotemia (elevated BUN and creatinine), and retention of sodium and water.

Hematuria: Damage to the glomerular capillary wall allows red blood cells to pass through, producing dysmorphic RBCs (distorted as they squeeze through the damaged basement membrane) and RBC casts (formed when red cells aggregate in the renal tubules). The gross hematuria gives urine its characteristic "cola-colored" or "tea-colored" appearance.

Proteinuria: The disrupted filtration barrier also leaks protein, though typically in the subnephrotic range (less than 3.5 g/day in adults, or less than 40 mg/m$^2$/hr in children). This distinguishes nephritic from nephrotic syndrome at the clinical level.

Edema and hypertension: The primary mechanism of edema in nephritic syndrome is salt and water retention due to decreased GFR, not hypoalbuminemia (as in nephrotic syndrome). This volume expansion also drives hypertension. The edema characteristically affects the periorbital region and is worse upon waking because the loose periorbital connective tissue readily accumulates interstitial fluid when the patient is supine overnight.

On electron microscopy, the hallmark finding is subepithelial "humps" or dense deposits that appear as dome-shaped electron-dense deposits on the outer surface of the GBM. On light microscopy, there is diffuse endocapillary proliferation with neutrophil infiltration. On immunofluorescence, a "starry sky" or "lumpy-bumpy" granular pattern of IgG and C3 deposition is seen along the capillary walls and mesangium.

2. Classification and Clinical Manifestation

Presentation Category	Clinical Features	Frequency
Subclinical (Asymptomatic)	Microscopic hematuria and/or mild proteinuria detected only on urinalysis; low C3; no symptoms	Most common form overall (up to 50% of cases during epidemics)
Classic Acute Nephritic Syndrome	Gross hematuria (cola-colored urine), periorbital/facial edema, oliguria, hypertension, mild to moderate proteinuria	Most common symptomatic presentation
Nephritic Syndrome with Nephrotic-Range Proteinuria	Features of nephritic syndrome plus heavy proteinuria (>3.5 g/day or >40 mg/m$^2$/hr), may have significant hypoalbuminemia and more pronounced edema	Uncommon (approximately 5% of cases)
Rapidly Progressive (Crescentic) Glomerulonephritis	Rapid decline in renal function over days to weeks; crescent formation on biopsy	Rare but serious; more common in adult-onset PSGN
Hypertensive Encephalopathy / Pulmonary Edema	Severe hypertension leading to headache, seizures, altered mental status; or acute pulmonary edema from volume overload	Rare complications of severe PSGN

By Preceding Infection Type

Preceding Infection	Latent Period	Serologic Marker	Geographic/Seasonal Pattern
Pharyngitis	1 to 3 weeks	Elevated ASO titer	Temperate climates; winter and spring
Skin infection (Impetigo/Pyoderma)	3 to 6 weeks	Elevated anti-DNase B (ASO often normal)	Tropical climates; summer and autumn

3. Diagnostic Workup

Test	Purpose	Expected Finding
Urinalysis with microscopy	Best initial test to detect nephritis	Hematuria (gross or microscopic), dysmorphic RBCs, RBC casts, proteinuria (usually subnephrotic)
Serum C3 complement	Supports immune complex-mediated GN; helps narrow the differential	Low C3 (returns to normal within 6 to 8 weeks); C4 is typically normal (alternative pathway activation)
ASO titer	Evidence of recent streptococcal pharyngitis	Elevated (peaks 3 to 5 weeks after pharyngeal infection)
Anti-DNase B	Evidence of recent streptococcal skin infection	Elevated (more reliable than ASO for impetigo)
Streptozyme test	Broad screening panel for multiple anti-streptococcal antibodies	Positive; useful when ASO alone is negative
Serum BUN and creatinine	Assess renal function	Elevated (reflecting decreased GFR)
Throat or skin culture	Identify residual GAS infection	May be positive, but often the infection has resolved by the time nephritis appears
Renal biopsy	Gold standard for definitive histological diagnosis	Diffuse endocapillary proliferative GN on light microscopy; "lumpy-bumpy" granular IgG/C3 on immunofluorescence; subepithelial humps on electron microscopy

The best initial test is a urinalysis with microscopy. When a child presents with periorbital edema and dark urine after a recent sore throat, the urinalysis will reveal hematuria (often gross), proteinuria, and the critical finding of RBC casts. The presence of RBC casts localizes the bleeding to the glomerulus, confirming glomerulonephritis rather than a lower urinary tract source.

The next step is to check serum complement levels, particularly C3. A low C3 with normal C4 is characteristic of PSGN and reflects activation of the alternative complement pathway. This finding is extremely high-yield because it narrows the differential significantly. If C3 remains low beyond 8 weeks, you must reconsider the diagnosis and suspect conditions like membranoproliferative glomerulonephritis (MPGN) or C3 glomerulopathy, where hypocomplementemia persists.

Simultaneously, you should obtain streptococcal serologies to establish evidence of a preceding GAS infection. For pharyngitis, the ASO titer is the test of choice. For impetigo, ASO is often not elevated, and you should rely on anti-DNase B instead. These titers confirm that a streptococcal infection preceded the nephritic episode, which is essential for diagnosis, since the culture may already be negative by the time kidney symptoms appear.

Renal biopsy is not routinely performed in PSGN. Most cases in children follow a classic, self-limiting course, and the clinical and serologic picture is sufficient for diagnosis. Biopsy is reserved for atypical presentations:

C3 remains depressed beyond 8 weeks
Renal function progressively worsens instead of improving
Nephrotic-range proteinuria dominates the clinical picture
The patient is an adult (where the prognosis is less predictable)
There is clinical suspicion for an alternative diagnosis (e.g., IgA nephropathy, lupus nephritis)

When biopsy is performed, the most accurate (confirmatory) test reveals the triad of diffuse proliferative GN on light microscopy, granular IgG/C3 on immunofluorescence, and subepithelial electron-dense deposits ("humps") on electron microscopy.

4. Management and Treatment

Intervention	Details
Sodium and fluid restriction	Limit sodium intake to reduce edema and hypertension; restrict fluids if oliguric
Loop diuretics	Furosemide 1 to 2 mg/kg/dose IV or PO every 6 to 12 hours; first-line for managing volume overload and hypertension
Antihypertensives	If blood pressure remains elevated despite diuretics: calcium channel blockers (amlodipine 0.1 to 0.2 mg/kg/day PO) or hydralazine for acute episodes; avoid ACE inhibitors acutely if hyperkalemia is present
Antibiotic therapy	Penicillin V 250 mg PO twice daily (children <27 kg) or 500 mg PO twice daily (children >27 kg) for 10 days; or amoxicillin 50 mg/kg/day divided twice daily for 10 days; purpose is to eradicate residual streptococcal colonization
Dialysis	Reserved for severe cases with life-threatening hyperkalemia, refractory pulmonary edema, or uremic encephalopathy
Immunosuppressive therapy	Not indicated in typical PSGN; considered only if biopsy reveals crescentic GN

PSGN management is primarily supportive. There is no treatment that shortens the disease course or reverses the immune-mediated glomerular injury once it has occurred. The goal is to manage complications while the disease self-resolves.

Step 1: Volume and sodium management. Because edema and hypertension in PSGN result from salt and water retention (not from hypoalbuminemia as in nephrotic syndrome), the first intervention is restricting sodium and fluid intake. This alone can significantly improve mild edema and blood pressure.

Step 2: Loop diuretics. Furosemide is the diuretic of choice. It promotes natriuresis and diuresis, helping to relieve edema and lower blood pressure. The dose is 1 to 2 mg/kg/dose, given intravenously for moderate to severe cases or orally for milder presentations, every 6 to 12 hours as needed. Thiazide diuretics are generally avoided because they are less effective when GFR is significantly reduced.

Step 3: Antihypertensive therapy. If blood pressure remains elevated despite fluid restriction and diuretics, add a second-line antihypertensive. Calcium channel blockers like amlodipine are commonly used. For hypertensive urgency or encephalopathy, IV hydralazine (0.2 to 0.6 mg/kg/dose) or IV nicardipine infusion may be necessary. ACE inhibitors and ARBs should be used cautiously acutely because the patient may have hyperkalemia and reduced GFR; however, they may be considered later if significant proteinuria persists.

Step 4: Antibiotics. A full course of antibiotics is given to eradicate any residual GAS from the pharynx or skin, even though the infection may have already resolved. The rationale is twofold: to prevent spread of nephritogenic strains to close contacts and to eliminate any ongoing antigenic stimulation. Penicillin V for 10 days is standard. For penicillin-allergic patients, use a macrolide (azithromycin 12 mg/kg/day for 5 days, or erythromycin). Importantly, antibiotics do not alter the course of the glomerulonephritis itself and do not prevent PSGN if started after the latent period.

Step 5: Monitoring and follow-up. Most children recover completely. Hematuria resolves over weeks to months. Proteinuria may persist for up to a year. C3 complement should normalize within 6 to 8 weeks. A C3 that remains low beyond 8 weeks is a red flag requiring reevaluation and possible biopsy.

Prognosis: In children, the prognosis is excellent, with greater than 95% making a full recovery. In adults, PSGN carries a less favorable prognosis with a higher risk of persistent renal impairment.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
IgA Nephropathy (Berger Disease)	Both present with hematuria after an upper respiratory infection in a young patient	IgA nephropathy has a short latent period (hematuria occurs within 1 to 2 days, often simultaneously with the URI); complement levels are normal; the patient is typically an adolescent or young adult, not a young child; biopsy shows mesangial IgA deposits, not subepithelial humps
Membranoproliferative GN (MPGN)	Both cause nephritic syndrome with low C3	MPGN has persistently low C3 beyond 8 weeks (it does not self-correct); often presents in older children or adults; may have nephrotic-range proteinuria; biopsy shows "tram-tracking" of the GBM
Lupus Nephritis (SLE)	Both cause nephritic syndrome with low complement	Lupus nephritis has both low C3 and low C4 (classical pathway activation); positive ANA and anti-dsDNA antibodies; other systemic features (malar rash, arthralgia, serositis); more common in adolescent and young adult females
Henoch-Schonlein Purpura (IgA Vasculitis) Nephritis	Both occur in children and can present with hematuria and proteinuria	HSP has palpable purpura on the buttocks and lower extremities, abdominal pain, and arthralgia; complement levels are normal; biopsy shows mesangial IgA (similar to IgA nephropathy)
Alport Syndrome	Both cause hematuria in children	Alport syndrome is a hereditary condition with persistent hematuria, progressive sensorineural hearing loss, and ocular abnormalities (anterior lenticonus); complement is normal; no preceding infection; biopsy shows GBM splitting ("basket weave" on EM)
Hemolytic Uremic Syndrome (HUS)	Both occur in children and may present with decreased urine output and renal failure	HUS classically follows a diarrheal illness (E. coli O157:H7), not a streptococcal infection; microangiopathic hemolytic anemia (schistocytes on smear) and thrombocytopenia are present; complement is usually normal in typical HUS
Nephrotic Syndrome (Minimal Change Disease)	Both present with edema in a child	Nephrotic syndrome presents with generalized (dependent) edema, heavy proteinuria (>3.5 g/day), hypoalbuminemia, and hyperlipidemia; urine sediment is bland (no RBC casts); complement is normal

6. Traps and High-Yield Pearls

The most common way students lose points on PSGN questions is by confusing PSGN with IgA nephropathy. Both conditions present as hematuria following an upper respiratory infection in a young patient, and test-writers deliberately exploit this overlap. The discriminator is the latent period: PSGN has a gap of 1 to 3 weeks between pharyngitis and renal symptoms, while IgA nephropathy presents with synpharyngitic hematuria (hematuria occurring simultaneously with or within 1 to 2 days of the URI). If the vignette says the patient develops hematuria "during" or "immediately after" a sore throat, think IgA nephropathy. If there is a clear interval of weeks, think PSGN.

A second common trap involves complement levels. Students may forget that PSGN causes a transient drop in C3, and that C3 should normalize by 6 to 8 weeks. When a vignette describes persistent hypocomplementemia beyond this window, the answer shifts to MPGN or C3 glomerulopathy. Test-writers use the timeline of complement normalization as a pivot point in follow-up questions.

A third pitfall is the assumption that antibiotics treat or prevent PSGN. Antibiotics eradicate the organism and prevent spread, but they do not reverse the glomerulonephritis or prevent it if given after the latent period has begun. Questions may present a scenario asking whether early antibiotic therapy during pharyngitis prevents PSGN. The answer is no, unlike rheumatic fever, which can be prevented by timely antibiotic treatment.

Students also sometimes confuse the mechanism of edema in nephritic versus nephrotic syndrome. In PSGN (nephritic), the edema is driven by salt and water retention from reduced GFR, and the treatment is diuretics and sodium restriction. In nephrotic syndrome, the edema is driven by hypoalbuminemia and reduced oncotic pressure. A question that asks the "most likely mechanism of edema" in a child with cola-colored urine, RBC casts, and low C3 is testing whether you recognize the nephritic mechanism.

Finally, remember that PSGN is not an indication for immunosuppressive therapy (steroids, cyclophosphamide). This is a self-limited immune complex disease in children. Immunosuppression is reserved for crescentic (rapidly progressive) GN, which is a rare complication. A question offering steroids as a management option for typical PSGN is a distractor designed to test whether you understand the supportive nature of treatment.

The core competency being tested across all PSGN questions is pattern recognition: linking the triad of cola-colored urine + periorbital edema + low C3 to a child with a recent streptococcal infection, and then correctly sequencing the workup (urinalysis first, then complement and streptococcal serologies) and management (supportive care, not immunosuppression).

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