Step 1 topics → Renal → Post-Streptococcal Glomerulonephritis (PSGN)

Post-Streptococcal Glomerulonephritis (PSGN)

USMLE Step 1 trap: Confuses PSGN's post-infectious latency with IgA nephropathy's synpharyngitic timing. PSGN presents 1–3 weeks after pharyngitis or 3–6 weeks after impetigo, reflecting the latency period for immune complex formation; IgA nephropathy occurs concurrently with infection.

Post-streptococcal glomerulonephritis is the classic example of immune complex-mediated nephritic syndrome. It follows infection with nephritogenic strains of Group A Streptococcus — either pharyngitis or impetigo — and results from subepithelial immune complex deposition that activates complement and drives glomerular inflammation. USMLE Step 1 loves this disease because it has distinct, testable findings at every level: clinical presentation, serology, light microscopy, immunofluorescence, and electron microscopy.

The exam tests PSGN from multiple angles. Pure recall questions ask you to identify the 'lumpy bumpy' immunofluorescence or the subepithelial humps on EM. Application questions give you a child with cola-colored urine and periorbital edema two weeks after a sore throat and ask you to pick the correct complement pattern or the most likely biopsy finding. Passage-based questions often bury the diagnosis in clinical details and ask you to differentiate PSGN from IgA nephropathy or lupus nephritis based on timing or lab values.

The three places students go wrong: they forget that PSGN is post-infectious (not concurrent), they confuse the complement pattern with lupus nephritis, and they misidentify the subepithelial humps as membranous nephropathy deposits. These are not arbitrary mistakes — they reflect gaps in understanding the underlying immune mechanisms, and USMLE Step 1 exploits all three of them.

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Common misconceptions

Common mistake
Wrong: PSGN presents during the active streptococcal infection, like IgA nephropathy.
Right: PSGN presents 1–3 weeks after pharyngitis or 3–6 weeks after impetigo, reflecting the latency period for immune complex formation; IgA nephropathy occurs concurrently with infection.
PSGN requires time for the immune response to develop — streptococcal antigens must be processed, antibodies must form, and immune complexes must deposit before glomerular injury occurs. This latency is 1–3 weeks after pharyngitis and 3–6 weeks after impetigo. IgA nephropathy, by contrast, is driven by pre-formed IgA that deposits almost immediately during mucosal infection — that's why hematuria appears concurrently or within days of illness. If a question tells you the strep infection is still active and the patient already has nephritis, think IgA, not PSGN.
Common mistake
Wrong: PSGN and lupus nephritis have the same complement consumption pattern.
Right: PSGN transiently lowers C3 with normal C1q and C4 (alternative pathway activation), while lupus nephritis lowers C3, C4, and C1q (classical pathway activation).
The pathway of complement activation determines which components are consumed. PSGN activates the alternative pathway (starting at C3), so only C3 is depleted — C1q and C4, which are upstream classical pathway components, remain normal. Lupus nephritis activates the classical pathway via immune complex binding to C1q, so it consumes C1q, C4, AND C3. On Step 1, if you see low C3 with normal C4 after a strep infection, that's PSGN. If you see low C3, low C4, and low C1q in a young woman with multi-organ involvement, that's lupus.
Common mistake
Wrong: The subepithelial 'humps' on EM in PSGN are the same as the deposits seen in membranous nephropathy.
Right: PSGN shows large, irregular subepithelial 'humps' (immune complexes) on EM that are transient and associated with nephritic syndrome, whereas membranous nephropathy shows uniform, small subepithelial deposits associated with nephrotic syndrome.
The location and context are completely different. PSGN's subepithelial humps are large, irregular, 'hump-shaped' electron-dense deposits — they're immune complexes that formed in the circulation and got trapped under the podocytes, triggering a nephritic response. Membranous nephropathy also has subepithelial deposits, but they're small, uniform, and arranged in a 'spike and dome' pattern on silver stain; they cause nephrotic syndrome, not nephritic. The key distinguishing features: PSGN humps are large and irregular, associated with nephritic syndrome, post-infectious, and transient; membranous deposits are uniform, associated with nephrotic syndrome, and persist.
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What the exam tests

  1. Know the classic PSGN presentation: a child (or adult) with sudden-onset hematuria, hypertension, periorbital edema, and oliguria following a streptococcal infection — and understand that the prognosis is excellent in children but worse in adults.
  2. Identify the three-layer biopsy picture: light microscopy shows hypercellular, enlarged glomeruli with neutrophil infiltration; immunofluorescence shows coarse 'lumpy bumpy' granular IgG and C3 deposits along the GBM; EM shows large, irregular subepithelial electron-dense 'humps.'
  3. Know that PSGN selectively lowers C3 while C1q and C4 remain normal, reflecting alternative complement pathway activation — and be able to contrast this with lupus nephritis, which lowers C3, C4, and C1q through classical pathway activation.
  4. Use timing to distinguish PSGN from IgA nephropathy: PSGN appears 1–3 weeks after pharyngitis or 3–6 weeks after skin infection; IgA nephropathy causes hematuria concurrently ('synpharyngitic') with the infection, often within 1–2 days.

Can you avoid these mistakes?

A 7-year-old boy presents with dark urine, puffy eyes, and high blood pressure 2 weeks after being treated for strep throat. Labs show low C3, normal C4, and RBC casts in urine. What complement pathway is activated, and how does this differ from what you'd expect in lupus nephritis?
You're shown an EM image of a kidney biopsy with large, irregular electron-dense deposits on the epithelial side of the glomerular basement membrane. What disease does this suggest, and why are these deposits NOT the same as those seen in membranous nephropathy?
A 19-year-old gets gross hematuria every time he gets a cold — onset is within 1–2 days of symptoms. Another patient gets hematuria 3 weeks after a sore throat. What diagnosis fits each patient, and what is the mechanistic reason for the timing difference?
A child recovering from impetigo develops nephritic syndrome 5 weeks later. ASO titer is negative but anti-DNase B is positive. What disease is this, and why might ASO be negative in skin infections specifically?

Related topics

Nephritic Syndrome — Pattern and Differential Kidney Development (Pro-/Meso-/Metanephros) Congenital Renal Anomalies Nephron Structure and Segments Renal Vasculature (Afferent / Efferent / Vasa Recta)

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