Nephritic Syndrome

Why Low serum C3 with a preserved C4 level is correct

• The vignette (tea-colored urine, periorbital edema, hypertension, RBC casts 1-3 weeks after strep pharyngitis) is classic post-streptococcal glomerulonephritis.
• Post-strep GN activates the alternative complement pathway, producing a depressed C3 with a preserved C4; positive ASO/anti-DNase B titers confirm recent streptococcal infection.
• Complement classification of GN: LOW C3 = post-strep, MPGN, lupus, cryoglobulinemia; NORMAL C3 = IgA nephropathy, anti-GBM, ANCA-associated vasculitis.

Why the others are wrong

• Positive anti-GBM antibody on serology — defines anti-GBM (Goodpasture) disease, a pulmonary-renal RPGN with linear IgG on biopsy and a normal C3; it does not follow strep pharyngitis.
• Positive c-ANCA targeting proteinase 3 — points to granulomatosis with polyangiitis (pauci-immune crescentic GN with sinopulmonary disease and normal complement), not a post-infectious GN in a child.
• Elevated total serum IgA antibody level — suggests IgA nephropathy, which causes synpharyngitic hematuria within days of a URI and has a normal C3, not the 1-3 week post-strep latent period seen here.
• Low serum C3 with a low C4 level — low complement fits a post-infectious GN, but depression of both C3 and C4 signals classical-pathway consumption by immune complexes, as in lupus nephritis, immune-complex MPGN, or cryoglobulinemia; post-strep GN characteristically lowers C3 through the alternative pathway while C4 is preserved.

Why Plasma volume expansion from renal sodium retention is correct

• Dark urine with dysmorphic red cells and red cell casts, hypertension, edema and a mildly raised creatinine 10 days after a febrile sore throat, with an elevated antistreptolysin O titer, is post-streptococcal glomerulonephritis (PSGN); it is a nephritic syndrome, so proteinuria is modest (1+) and the serum albumin (3.9 g/dL) is normal.
• The inflamed, hypercellular glomeruli filter less, so the GFR and the filtered sodium load fall, but the tubules keep reabsorbing sodium avidly (glomerulotubular imbalance); sodium and water the kidney should excrete are held back, and the fractional excretion of sodium is usually below 1%.
• The retained salt and water expand the plasma and extracellular volume: higher capillary hydrostatic pressure pushes fluid into loose tissue such as the eyelids, the larger circulating volume raises blood pressure, and the 2-kg weight gain is the retained fluid. Renin and aldosterone are suppressed and atrial natriuretic peptide is raised, which marks the hypertension as volume-dependent (overfill).

Why the others are wrong

• Low plasma oncotic pressure from urinary protein loss - oncotic edema needs heavy, nephrotic-range proteinuria that drives the serum albumin below about 3 g/dL, as in minimal change disease; this child has 1+ protein and a normal albumin of 3.9 g/dL, and fluid drawn out of the vessels by a low oncotic pressure tends to shrink the circulating volume rather than raise blood pressure (the trap is blaming every renal edema on protein loss).
• Renin release triggered by reduced renal perfusion - hypertension in PSGN is low-renin: the expanded volume suppresses plasma renin activity and aldosterone, so the pressure is driven chiefly by volume rather than by angiotensin II; renin-driven renal hypertension is the pattern of renal artery stenosis, where the kidney beyond the narrowing is underperfused (the trap is assuming every renal cause of hypertension runs through renin).
• Capillary leak caused by circulating immune complexes - the streptococcal antigens and immune complexes lodge in and inflame the glomerulus, not capillaries throughout the body, so PSGN does not cause a generalized leak; a leak would also move plasma out of the vessels and tend to lower blood pressure, whereas edema together with hypertension points to an expanded circulating volume (the trap is extending the glomerular immune-complex injury to every capillary bed).
• Cardiac pump failure from acute rheumatic carditis - acute rheumatic fever is another nonsuppurative complication of strep throat and its carditis can cause heart failure, but it is not a glomerular disease, so it cannot explain red cell casts and dysmorphic red cells, nor does carditis account for hypertension; edema with hypertension points to an expanded circulating volume (the trap is confusing the two post-streptococcal syndromes).

Additional high-yield points

• Nephritic versus nephrotic edema: edema with hypertension, modest proteinuria and a near-normal albumin means an overfilled circulation with suppressed renin (PSGN); edema with nephrotic-range proteinuria and an albumin below 3 g/dL (minimal change disease in children) can come with a contracted circulating volume and a high renin (underfill), although primary renal sodium retention contributes there too.
• A low fractional excretion of sodium does not prove prerenal azotemia: acute glomerulonephritis also gives a FENa below 1%, because filtration falls while tubular sodium reabsorption stays intact; red cell casts point to the glomerulus.
• The antistreptolysin O titer usually rises after streptococcal pharyngitis, but the ASO response is often weak after impetigo, so anti-DNase B is the better marker of a recent streptococcal skin infection.