Edema in nephrotic syndrome is primarily caused by:

Question

BlackTom AI · Accepted Answer

Plasma oncotic pressure (largely determined by albumin) falls in nephrotic syndrome because massive proteinuria causes hypoalbuminemia; this reduces the inward (reabsorptive) Starling force and so favors net fluid filtration into the interstitium, producing edema.

1. Relevant knowledge points and concepts
- Nephrotic syndrome → heavy urinary loss of albumin → hypoalbuminemia.
- Starling forces govern fluid exchange: net filtration ∝ (capillary hydrostatic pressure − interstitial hydrostatic pressure) − (capillary oncotic pressure − interstitial oncotic pressure).
- Reduced capillary (plasma) oncotic pressure decreases the opposing force to filtration, so more fluid leaves capillaries into tissues.
- Secondary mechanisms (renal sodium and water retention) can exacerbate edema but are not the primary initiating cause.

2. Step-by-step reasoning
- In nephrotic syndrome albumin is lost in urine → plasma albumin concentration falls.
- Plasma oncotic pressure (πc) is proportional to plasma protein concentration; as albumin ↓, πc ↓.
- According to Starling relation, a ↓πc reduces the (πc − πi) term, so net filtration increases.
- Increased net filtration means fluid accumulates in interstitial spaces → edema.

3. Illustrative equation and intermediate steps
- Starling-type balance: Net filtration = Kf[(Pc − Pi) − σ(πc − πi)].
- If πc decreases, then (πc − πi) becomes smaller → the subtractive term is reduced → overall bracketed expression increases → net filtration increases.
- No numeric values needed; the sign change is the key.

4. Why the correct answer is correct
- Option c states “Reduced plasma oncotic pressure due to urinary protein loss,” which directly names the chain (proteinuria → hypoalbuminemia → ↓oncotic pressure → edema) and matches the physiological mechanism above.

5. Why other options are incorrect
- a. Increased erythropoietin production: Erythropoietin affects red cell production, not interstitial fluid balance.
- b. Destruction of alveolar walls: This pertains to lung parenchyma (e.g., emphysema) and is unrelated to peripheral edema in nephrotic syndrome.
- d. Increased arterial inflow to tissues: That would raise hydrostatic pressure locally, but nephrotic edema is driven primarily by loss of plasma oncotic pressure; increased arterial inflow is not the initiating mechanism.

6. Conclusion
The primary cause of edema in nephrotic syndrome is reduced plasma oncotic pressure from urinary protein loss; this confirms answer c.

Edema in nephrotic syndrome is primarily caused by:单项选择题

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